win: Rename clang_x64 to win_clang_x64 in build/toolchain/win, step 4/5.

In cross builds of chrome/win, the host and target toolchains currently
have the same name.  To fix this, rename clang_x64 to win_clang_x64.
Because the toolchain name is also referenced in libyuv, this requires
a five-sided change:

1. Introduce variable containing the toolchain name in src.git
2. Change libyuv to refer to the variable
3. Rename toolchain in src.git (including in the newly introduced var)
4* Let libyuv refer to the new name directly
5. Remove variable again

(See also https://codereview.chromium.org/2463143002)

Bug: 748501
Change-Id: I89fdf1503f1a57992a8336026d4c8d767685d53f
Reviewed-on: https://chromium-review.googlesource.com/585306
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>

BUILD.gn

@@ -7,7 +7,6 @@
 # be found in the AUTHORS file in the root of the source tree.
 
 import("libyuv.gni")
-import("//build/toolchain/win/clang_name.gni")
 import("//testing/test.gni")
 
 declare_args() {
@@ -48,7 +47,7 @@ group("libyuv") {
   if (is_win && target_cpu == "x64") {
     # Compile with clang in order to get inline assembly
     public_deps = [
-      ":libyuv_internal($win_clang_x64_toolchain)",
+      ":libyuv_internal(//build/toolchain/win:win_clang_x64)",
     ]
   } else {
     public_deps = [

DEPS

@@ -14,7 +14,7 @@ vars = {
 
 deps = {
   'src/build':
-    Var('chromium_git') + '/chromium/src/build' + '@' + '15b41f483525d2e036a529e12518e55d2e5182a8',
+    Var('chromium_git') + '/chromium/src/build' + '@' + '5a3f439e92d3a55858c4eeda57917664657f45fb',
   'src/buildtools':
     Var('chromium_git') + '/chromium/buildtools.git' + '@' + '5ad14542a6a74dd914f067b948c5d3e8d170396b',
   'src/testing':