Look for a test_expectations_w3c.txt file for import overrides, other w3c fixes.

This patch modifies run-webkit-tests to look for a test_expectations_w3c.txt
overrides file in src/webkit/tools/layout_tests that will be treated like
the skia overrides (used both upstream and downstream). Since the W3C test
repos are rolled into Chromium, it's possible that a new roll will cause
tests to fail on both sets of bots. We don't use the Skia file to avoid
confusion. Any failures added to that file should be immediately upstreamed
into LayoutTests/TestExpectations, and then removed from the downstream
file once Blink has rolled.

Also, modify the import script to automatically copy over any 'resources'
directory, fix the base offset for importing repos, and make the testharness.js-based-test detect more resilient
to whitespace at the beginning and end of the test.

With these fixes, we should be able to start importing tests from the W3C
repos for real.

R=eseidel@chromium.org, abarth@chromium.org, jamesr@chromium.org, ojan@chromium.org
BUG=242329

Review URL: https://codereview.chromium.org/93933015

git-svn-id: svn://svn.chromium.org/blink/trunk@164251 bbb929c8-8fbe-4397-9dbb-9b2b20218538