gn: Pass through msan flag.

BUG=none
TEST=GYP_DEFINES=msan=1 build/gyp_chromium works.
TBR=brettw@chromium.org

Review URL: https://codereview.chromium.org/136973012

git-svn-id: http://src.chromium.org/svn/trunk/src/build@246188 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

gyp_chromium

@@ -211,6 +211,7 @@ def GetArgsStringForGN(supplemental_files):
       ('use_goma', '1', 'use_goma=true'),
       ('asan', '1', 'is_asan=true'),
       ('lsan', '1', 'is_lsan=true'),
+      ('msan', '1', 'is_msan=true'),
       ('tsan', '1', 'is_tsan=true'),
   ]
   for i in remap_cases: