GN: Add support for prebuilt instrumented libraries.

build/config/compiler/BUILD.gn

Index: build/config/compiler/BUILD.gn
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index dbe2e5744993faa616fb6f8529a5f16e8d07d67b..fc728a044ed07c3b94b2af1c3bd0c6717cf7a8af 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -15,6 +15,7 @@ if (is_posix) {
 }
 
 import("//build/toolchain/ccache.gni")
+import("//build/config/sanitizers/sanitizers.gni")
 
 declare_args() {
   # Normally, Android builds are lightly optimized, even for debug builds, to
@@ -56,11 +57,6 @@ declare_args() {
     # The patch is preapplied to the internal toolchain and hence all bots.
     msvs_xtree_patched = false
   }
-
-  # Track where uninitialized memory originates from. From fastest to slowest:
-  # 0 - no tracking, 1 - track only the initial allocation site, 2 - track the
-  # chain of stores leading from allocation site to use site.
-  msan_track_origins = 2
 }
 
 # default_include_dirs ---------------------------------------------------------
@@ -175,7 +171,7 @@ config("compiler") {
         "-fsanitize-blacklist=$msan_blacklist_path",
       ]
 
-      # TODO(GYP): Support instrumented libraries and custom libc++.

[earthdok, 2015/05/28 18:27:45]

This isn't resolved. We need to support source builds in addition to binary
downloads.

It probably makes sense to decouple source builds from Chromium's build system
completely and support them only as a way to produce pre-built binaries for GCS.
But one way or another, this must be resolved before GYP builds stop working.

[Sam McNally, 2015/05/29 08:33:00]

Moved the TODO into the instrumented_libraries BUILD.gn.

+      # TODO(GYP): Support custom libc++.
     }
   }