DOM-object leak detection at run_webkit_tests.py

Now we're implementing the DOM-object leak detector. When the number of
DOM objects is increased after a test, we can assume that the test
causes a leak. This feature is available at content_shell with the flag
--enable-leak-detection.

This patch enables this leak detection feature at run_webkit_test.py
with the flag --enable-leak-detection.

content_shell is assumed to report a detected leak with the following
format:

  #LEAK - renderer pid PID (DETAIL_IN_JSON_FORMAT)

and run_webkit_test.py will treat this as a kind of error by this patch.

See also:
https://docs.google.com/a/chromium.org/document/d/1sFAsZxeISKnbGdXoLZlB2tDZ8pvO102ePFQx6TX4X14/edit

BUG=332630
TEST=test-webkitpy

Review URL: https://codereview.chromium.org/148153009

git-svn-id: svn://svn.chromium.org/blink/trunk@170255 bbb929c8-8fbe-4397-9dbb-9b2b20218538