ui/gfx: Add exclusion for testing/gmock to DEPS.

This is so we can prevent other from adding this dependency back.

They will think twice or at least consult with OWNERS if we have this in
place in the DEPS exclusion list.

BUG=134675
TEST=checkdeps
R=sky@chromium.org

Review URL: https://codereview.chromium.org/1138643012

Cr-Commit-Position: refs/heads/master@{#330392}

ui/gfx/DEPS

@@ -4,4 +4,6 @@ include_rules = [
   "+third_party/harfbuzz-ng",
   "+third_party/skia",
   "+ui/ios",
+
+  "-testing/gmock",
 ]