Use 'local-exec' for threadlocal_heap_ in tcmalloc.

third_party/tcmalloc/chromium/src/thread_cache.cc

Index: third_party/tcmalloc/chromium/src/thread_cache.cc
diff --git a/third_party/tcmalloc/chromium/src/thread_cache.cc b/third_party/tcmalloc/chromium/src/thread_cache.cc
index 1ad0f6d66d675011b7ec7db74a6f5b1a0afdf9b7..6818ce02c60c981644f41e6c61cbd2cd72c04da7 100644
--- a/third_party/tcmalloc/chromium/src/thread_cache.cc
+++ b/third_party/tcmalloc/chromium/src/thread_cache.cc
@@ -67,7 +67,12 @@ __thread ThreadCache* ThreadCache::threadlocal_heap_
 // See comments in thread_cache.h about this. Bug here:
 // http://code.google.com/p/chromium/issues/detail?id=124489
 #if defined(HAVE___ATTRIBUTE__) && !defined(PGO_GENERATE)
+// If we are not using component build, we can further optimize it.
+#if !defined(COMPONENT_BUILD)
+   __attribute__ ((tls_model ("local-exec")))
+#else
    __attribute__ ((tls_model ("initial-exec")))
+#endif
 # endif
    ;
 #endif