Fix V8UnitTest to also check the src root for src files.

The js2gtest rule with test_type = "unit" relies on this behavior
when gen_include_files are used - but until now it didn't work.

Bug: 947036
Change-Id: Ieed195c9be9d3c0873e66592d99ace64ccdf8b10
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1543529
Reviewed-by: David Tseng <dtseng@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: A Olsen <olsen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#646099}