Revert "Reland "[ios] Disable custom libcxx with xcode clang.""

This reverts commit 334e5ddc18f9ea2e073442a0bf327b6ee9512f08.

Reason for revert: broke two iOS bots (even though the same ones succeeded in the CQ).

Original change's description:
> Reland "[ios] Disable custom libcxx with xcode clang."
> 
> This reverts commit 7036423b744ca5996aad613d0f3dea30410f78dc.
> 
> Reason for revert: Now with a fix to cronet.
> 
> This isn't necessary with Xcode 10.2, but we are still stuck on
> 10.1.  For some reason the combination of 10.1 and
> use_custom_libcxx=false on iOS requires this extra .get()
> 
> Original change's description:
> > Revert "[ios] Disable custom libcxx with xcode clang."
> >
> > This reverts commit 684d7a436007ccd4da585e16a651f7358271efc1.
> >
> > Reason for revert: Broke the tree.
> >
> > Original change's description:
> > > [ios] Disable custom libcxx with xcode clang.
> > >
> > > The combination of Xcode clang, a custom libcxx and gmock is causing crashes
> > > in our tests.
> > >
> > > Disabling custom libcxx with xcode clang for now.
> > >
> > > Bug: 956995
> > > Change-Id: Ia32a5d77d9ab7810d7c09c258c91dd60544f4747
> > > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1588363
> > > Auto-Submit: Justin Cohen <justincohen@chromium.org>
> > > Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
> > > Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
> > > Commit-Queue: Justin Cohen <justincohen@chromium.org>
> > > Cr-Commit-Position: refs/heads/master@{#655205}
> >
> > TBR=thakis@chromium.org,justincohen@chromium.org,olivierrobin@chromium.org,thomasanderson@chromium.org
> >
> > Change-Id: I0ebc8232d8e3fcea3f5a96552e58ac766f3d5888
> > No-Presubmit: true
> > No-Tree-Checks: true
> > No-Try: true
> > Bug: 956995
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1589704
> > Reviewed-by: Justin Cohen <justincohen@chromium.org>
> > Commit-Queue: Justin Cohen <justincohen@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#655209}
> 
> TBR=thakis@chromium.org,justincohen@chromium.org,olivierrobin@chromium.org,thomasanderson@chromium.org
> 
> Change-Id: I555add1b6626b6d6acfc536a8416a0b11822c3ed
> Bug: 956995
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1588852
> Commit-Queue: Justin Cohen <justincohen@chromium.org>
> Commit-Queue: Misha Efimov <mef@chromium.org>
> Commit-Queue: Nico Weber <thakis@chromium.org>
> Reviewed-by: Misha Efimov <mef@chromium.org>
> Reviewed-by: Nico Weber <thakis@chromium.org>
> Reviewed-by: Justin Cohen <justincohen@chromium.org>
> Auto-Submit: Justin Cohen <justincohen@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#655358}

TBR=thakis@chromium.org,mef@chromium.org,justincohen@chromium.org,olivierrobin@chromium.org,thomasanderson@chromium.org

Change-Id: I3b22f8edf3c83402e485bcd3dd6f37e77bf38359
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 956995, 958185
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1590232
Reviewed-by: Carlos Knippschild <carlosk@chromium.org>
Commit-Queue: Carlos Knippschild <carlosk@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#655408}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 6eb40789c00d5d95d1286b4875d58623ddc68624

config/c++/c++.gni

@@ -11,7 +11,7 @@ declare_args() {
   # Don't check in changes that set this to false for more platforms; doing so
   # is not supported.
   use_custom_libcxx =
-      is_fuchsia || is_android || is_mac || (is_ios && !use_xcode_clang) ||
+      is_fuchsia || is_android || is_mac || is_ios ||
       (is_linux &&
        (!is_chromeos || default_toolchain != "//build/toolchain/cros:target"))