Move gtest/gmock BUILD.gn files to main tree.

Also moves the "unused variable warning" to a config in the build directory so
it can be shared with outher targets without duplication on the command line.

BUG=
R=cjhopman@chromium.org

Review URL: https://codereview.chromium.org/219433005

git-svn-id: http://src.chromium.org/svn/trunk/src/build@260949 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

config/win/BUILD.gn

@@ -109,3 +109,11 @@ config("incremental_linking") {
 config("no_incremental_linking") {
   ldflags = [ "/INCREMENTAL:NO" ]
 }
+
+# Disable unused variable warning ----------------------------------------------
+
+# Sometimes third-party code wants to disable this warning under MSVC, so this
+# config makes it easy to do so.
+config("disable_unused_variable_warning") {
+  cflags = [ "/wd4800" ]
+}