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

build/config/win/BUILD.gn

Index: build/config/win/BUILD.gn
diff --git a/build/config/win/BUILD.gn b/build/config/win/BUILD.gn
index 3175b163bb70d093df607f6cd3200e278b9f50dc..fdc26a72e44b8c83652e4a90c04ac6502aa4eb19 100644
--- a/build/config/win/BUILD.gn
+++ b/build/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" ]
+}