MSan: introduce a GYP flag to control origin tracking.

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index 166130c6e231d0f813b5d306ed1db1ff91661451..d94cb484d8a6f715332432182a49e929f4078a35 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -353,7 +353,8 @@
       # See https://sites.google.com/a/chromium.org/dev/developers/testing/addresssanitizer
       'asan%': 0,
       # Enable coverage gathering instrumentation in ASan. This flag also
-      # controls coverage granularity (experimental).
+      # controls coverage granularity (1 for function-level coverage, 2 for
+      # block-level coverage).
       'asan_coverage%': 0,
 
       # Enable Chromium overrides of the default configurations for various
@@ -380,6 +381,10 @@
       # See http://clang.llvm.org/docs/MemorySanitizer.html
       'msan%': 0,
       'msan_blacklist%': '<(PRODUCT_DIR)/../../tools/msan/blacklist.txt',
+      # 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%': 1,
 
       # Enable building with UBSan (Clang's -fsanitize=undefined option).
       # -fsanitize=undefined only works with clang, but ubsan=1 implies clang=1
@@ -1066,6 +1071,7 @@
     'lsan%': '<(lsan)',
     'msan%': '<(msan)',
     'msan_blacklist%': '<(msan_blacklist)',
+    'msan_track_origins%': '<(msan_track_origins)',
     'tsan%': '<(tsan)',
     'tsan_blacklist%': '<(tsan_blacklist)',
     'ubsan%': '<(ubsan)',
@@ -3977,7 +3983,7 @@
               ['_toolset=="target"', {
                 'cflags': [
                   '-fsanitize=memory',
-                  '-fsanitize-memory-track-origins',
+                  '-fsanitize-memory-track-origins=<(msan_track_origins)',
                   '-fPIC',
                   '-fsanitize-blacklist=<(msan_blacklist)',
                 ],