Use instrumented libc++ by default in TSan builds.

BUG=318770
TBR=earthdok@chromium.org

Review URL: https://codereview.chromium.org/181013015

git-svn-id: http://src.chromium.org/svn/trunk/src/build@262558 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

common.gypi

@@ -1909,6 +1909,7 @@
       }],
       ['tsan==1', {
         'clang%': 1,
+        'use_custom_libcxx%': 1,
       }],
       ['msan==1', {
         'clang%': 1,