For GYP build, V8 configures gtest and gmock in its //testing, and OTOH for
GN build, it imports BUILD.gn from chromium and uses other configurations
from its own. However, a recent chromium change on the BUILD.gn requires
//testing update too. That prevents //build roll of V8.

BUG=chromium:630299

Review-Url: https://codereview.chromium.org/2179743002
Cr-Commit-Position: refs/heads/master@{#38021}