Make PRESUBMIT.py and tests in LayoutTests/ workable in web_tests/.

This CL makes LayoutTests/PRESUBMIT.py and some tests in LayoutTests/
workable in blink/web_tests/ too.
Moving LayoutTests directory gets easier by this CL.

* PRESUBMIT.py
 - Use PresubmitLocalPath() instead of 'LayoutTests'
 - Remove unnecessary check for existence of 'LayoutTests' in
  AffectedFiles(), which returns files under the PRESUBMIT directory.
  _FindNewViolationsOfRule() also handles only files under the
  PRESUBMIT directory.

* Tests
 - Update code searching for 'LayoutTests' so that it searches for
   'web_tests' too.
 - Drop 'LayoutTests' from test expectations if we can
 - Introduce relativeTestPath()

Change-Id: I741ab3cb29d8e6c2a35e3b7669f92744f76c2839
Bug: 843412
Reviewed-on: https://chromium-review.googlesource.com/c/1276047
Reviewed-by: Quinten Yearsley <qyearsley@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599068}
23 files changed