GN: Add support for prebuilt instrumented libraries.

build/config/sanitizers/sanitizers.gni

Index: build/config/sanitizers/sanitizers.gni
diff --git a/build/config/sanitizers/sanitizers.gni b/build/config/sanitizers/sanitizers.gni
index e9aeeae0492763879637d8b93e6558282e5fbab8..a7b965824a71455e3eba962e082f9901cf905995 100644
--- a/build/config/sanitizers/sanitizers.gni
+++ b/build/config/sanitizers/sanitizers.gni
@@ -7,4 +7,14 @@ declare_args() {
   # buildtools/third_party/libc++abi) instead of stdlibc++ as standard library.
   # This is intended to be used for instrumented builds.
   use_custom_libcxx = (is_asan && is_linux) || is_tsan || is_msan
+
+  # 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
+
+  # Use dynamic libraries instrumented by one of the sanitizers instead of the
+  # standard system libraries. Set this flag to download prebuilt binaries from
+  # GCS.
+  use_prebuilt_instrumented_libraries = false
 }