Clean up accessibility enums for use in Chromium.

The goal of this change is to make it possible for Chromium to use
Blink accessibility role, state, and event enums throughout
its codebase, rather than copying to separate enums.

* Rename the enums in public/web so they're more concise, e.g.
    WebAccessibilityRoleDocumentArticle -> WebAXTypes::ArticleRole
    WebAccessibilityNotificationLoadComplete -> WebAXTypes::LoadComplete
* Add missing enum values currently used in Chromium, delete unused values.
* Rename a few enum values that were unclear or confusing.

This will require a cleanup change after Chromium switches over.

BUG=269034

Review URL: https://chromiumcodereview.appspot.com/22331005

git-svn-id: svn://svn.chromium.org/blink/trunk@156740 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Increase mediasource-play per-test timeout

The per-test timeout defaults to 2 seconds in testharness.js.
Sometimes this test takes longer, leading to flaky results.
This change increases this test's timeout from the default 2 seconds
to 5 seconds, and removes the timeout expectation.

R=acolwell@chromium.org
TEST=timeout no longer repros locally on linux Debug across --iterations=10 --repeat-each=10
BUG=278400

Review URL: https://chromiumcodereview.appspot.com/23452002

git-svn-id: svn://svn.chromium.org/blink/trunk@156741 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Fix svg/masking/* tests.

"rx" is not a valid <circle> attribute - these tests were not drawing anything.

Also switching to rects to avoid reftest antialiasing issues.

For non-alpha variants, using a 50% luminance to capture the effect on the
output color.

R=pdr@chromium.org,dschulze@chromium.org

Review URL: https://chromiumcodereview.appspot.com/23485004

git-svn-id: svn://svn.chromium.org/blink/trunk@156742 bbb929c8-8fbe-4397-9dbb-9b2b20218538