Revert "Fix android generate_linker_map check."

This reverts commit a2cb14b2badc836a161f16eda1ad72bf1d020887.

Reason for revert: Apparently breaks CrOS builds, see comments on review.

Original change's description:
> Fix android generate_linker_map check.
> 
> This change fixes gn refusing to build when `generate_linker_map` is
> true for android targets from a linux host.
> 
> Bug: 986842
> Change-Id: Ieb5e4d236ca54c87cf5f09fe26ce82a9bfe3191f
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1715024
> Reviewed-by: Dirk Pranke <dpranke@chromium.org>
> Commit-Queue: Nina Satragno <nsatragno@chromium.org>
> Auto-Submit: Nina Satragno <nsatragno@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#681105}

TBR=dpranke@chromium.org,nsatragno@chromium.org

Change-Id: I9b0a81fe63a79fe5be29cfb1a08be953ce704c7a
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 986842
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1720125
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#681246}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 236a79bd2a4689fbf4ccb59d6ba289d2716cd90b

toolchain/toolchain.gni

@@ -30,7 +30,7 @@ if (generate_linker_map) {
   assert(
       is_official_build,
       "Linker map files should only be generated when is_official_build = true")
-  assert(target_os == "android" || target_os == "linux",
+  assert(current_os == "android" || target_os == "linux",
          "Linker map files should only be generated for Android and Linux")
 }