[webkitpy] Treat LayoutNG name mismatch as PASS

This patch changes the layout test runner to treat LayoutNG name
mismatch (e.g., LayoutBlockFlow vs. LayoutNGBlockFlow) as PASS instead
of TEXT failure, to reduce noise in test result.

Bug: 795043
Change-Id: Ia0a358f7ccf47cc809fff6b20eea1847d4754413
Reviewed-on: https://chromium-review.googlesource.com/828276


Reviewed-by: Aleks Totic <atotic@chromium.org>
Reviewed-by: Quinten Yearsley <qyearsley@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524407}