Mark gtest and gmock as testonly in GN.

build/secondary/testing/gtest/BUILD.gn

Index: build/secondary/testing/gtest/BUILD.gn
diff --git a/build/secondary/testing/gtest/BUILD.gn b/build/secondary/testing/gtest/BUILD.gn
index cab98923969e319ce86e846e2b70e5e1d723dbf2..81fcc143400edb27ca93ce053565fae581e9bdd5 100644
--- a/build/secondary/testing/gtest/BUILD.gn
+++ b/build/secondary/testing/gtest/BUILD.gn
@@ -59,6 +59,8 @@ config("gtest_direct_config") {
 }
 
 static_library("gtest") {
+  # TODO http://crbug.com/412064 enable this flag all the time.
+  testonly = !is_component_build
   sources = [
     "include/gtest/gtest-death-test.h",
     "include/gtest/gtest-message.h",
@@ -111,6 +113,8 @@ static_library("gtest") {
 }
 
 source_set("gtest_main") {
+  # TODO http://crbug.com/412064 enable this flag all the time.
+  testonly = !is_component_build
   sources = [ "src/gtest_main.cc" ]
   deps = [ ":gtest" ]
 }