Have test_content_client look for .pak file in the .apk as well as on disk

Most tests use .isolate files to put files on disk, but
webkit_unit_tests (maybe others?) don't.

BUG=505538

Review URL: https://codereview.chromium.org/1217063002

Cr-Commit-Position: refs/heads/master@{#336644}
1 file changed