Create gpu_tests version of GetChromiumSrcDir.

In preperation for removing GetChromiumSrcDir from Telemetry, move all
usage in gpu_tests to a local version of the function.

BUG=511332

Review URL: https://codereview.chromium.org/1375303002

Cr-Commit-Position: refs/heads/master@{#351695}
7 files changed