Fix header dependency in WebKit/public about testing

This fixes disallowed inclusion of testing/gmock/include/gmock/gmock.h
in WebKit/public:blink_headers.

This is a prerequisite of whitelisting //third_party/WebKit/public
for header checking in GN.

BUG=716359

Review-Url: https://codereview.chromium.org/2853173002
Cr-Commit-Position: refs/heads/master@{#468560}
1 file changed