[webkitpy] Refactor diff_parser.py

Precompile all regexes to eliminate self developed regex caching.

BUG=none

Review URL: https://codereview.chromium.org/637503004

git-svn-id: svn://svn.chromium.org/blink/trunk@183448 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Retire fast/dom/Window/window-early-properties-xhr.html flaky expectation.

No evidence of it being slow on Windows any longer.

R=
BUG=409713
NOTRY=true

Review URL: https://codereview.chromium.org/643593002

git-svn-id: svn://svn.chromium.org/blink/trunk@183449 bbb929c8-8fbe-4397-9dbb-9b2b20218538