blinkpy and content_shell: Supports third_party/blink/web_tests

This CL adds third_party/blink/web_tests support to blinkpy and
content_shell.  If the path exists, they assume Web Test files are
in the directory.  Otherwise, Web Test files are in WebKit/LayoutTests
as ever.

We'll land a CL to move files from WebKit/LayoutTests to
blink/web_tests, then remove !TESTS_IN_BLINK code path.

* blinkpy:
  path_finder.py provides TESTS_IN_BLINK flag, RELATIVE_WEB_TESTS, and
  WEB_TESTS_LAST_COMPONENTS. They return values for "third_party/blink/
  web_tests" if the path actually exists.
  Other blinkpy code supports both of web_tests and LayoutTests by
  using these new values provided by path_finder.py.
  web_tests/port/base.py passs '--tests-in-blink' flag to content_shell
  if TESTS_IN_BLINK flags is true.

* content/shell:
  Add "--tests-in-blink" flag. It affects relative path resolution in
  command-line, and resolution for file:///tmp/LayoutTests/ URLs in
  tests.

Bug: 710535, 843412
Change-Id: Id4ba9a91bfa6f6985471b679e4a5c73a7bb5f111
Reviewed-on: https://chromium-review.googlesource.com/c/1260602
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Quinten Yearsley <qyearsley@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597797}
29 files changed