Land prep work to enable NaCl in the Linux x64 GN builds.

This should get most of NaCl and the PPAPI stuff needed
for NaCl building and linking. There is more work to be
done to get some of the test binaries working (and
probably fill out parts of the NaCl SDK) and possibly
some pnacl support work remaining as well.

NaCl is still disabled by default (set enable_nacl=true
to change). Enabling nacl is still mostly untested and
likely doesn't work at all :).

R=ncbray@chromium.org, brettw@chromium.org
BUG=432959
CQ_EXTRA_TRYBOTS=tryserver.chromium.linux:android_chromium_gn_compile_dbg,android_chromium_gn_compile_rel;tryserver.chromium.win:win8_chromium_gn_rel,win8_chromium_gn_dbg;tryserver.chromium.mac:mac_chromium_gn_rel,mac_chromium_gn_dbg

Review URL: https://codereview.chromium.org/877553008

Cr-Original-Commit-Position: refs/heads/master@{#318180}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 6065cf751340a8a99b670357b2053ed6df12a7af

config/BUILDCONFIG.gn

@@ -548,6 +548,12 @@ if (is_win) {
 } else if (is_ios) {
   host_toolchain = "//build/toolchain/mac:host_clang"
   set_default_toolchain("//build/toolchain/mac:clang")
+} else if (is_nacl) {
+  # TODO(GYP): This will need to change when we get NaCl working
+  # on multiple platforms, but this whole block of code (how we define
+  # host_toolchain) needs to be reworked regardless to key off of build_os
+  # and build_cpu_arch rather than the is_* variables.
+  host_toolchain = "//build/toolchain/linux:clang_x64"
 }
 
 # ==============================================================================

config/features.gni

@@ -22,9 +22,20 @@ declare_args() {
   enable_plugins = !is_android && !is_ios
 
   # Enables Native Client support.
-  # TODO(GYP) enable this when nacl works in GN.
+  # TODO(GYP): Get NaCl linking on other platforms.
+  # Also, see if we can always get rid of enable_nacl_untrusted and
+  # enable_pnacl and always build them if enable_nacl is true.
+  # The "is_nacl" part of the condition is needed to ensure that
+  # the untrusted code is built properly; arguably it should be
+  # guarded by "is_nacl" directly rather than enable_nacl_untrusted, but
+  # this will go away when Mac and Win are working and we can just use
+  # the commented out logic.
+  # enable_nacl = !is_ios && !is_android
+  # Currently this *should* work:
+  # enable_nacl = (is_linux && build_cpu_arch == "x64") || is_nacl
   enable_nacl = false
-  #enable_nacl = (!is_ios && !is_android)
+  enable_nacl_untrusted = enable_nacl
+  enable_pnacl = enable_nacl_untrusted
 
   # If debug_devtools is set to true, JavaScript files for DevTools are stored
   # as is and loaded from disk. Otherwise, a concatenated file is stored in