Document that the WebNode class and its subclasses must not have any virtual methods.

This requirement allows code using classes in this heirarchy to safely
static_cast to the appropriate subclass based on the actual type of the wrapped
node.

BUG=278295
TEST=none
R=jamesr@chromium.org

Review URL: https://chromiumcodereview.appspot.com/23453016

git-svn-id: svn://svn.chromium.org/blink/trunk@156933 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Rebaseline SVG mask/clip tests after r156928

TBR=pdr@chromium.org

Review URL: https://codereview.chromium.org/23608006

git-svn-id: svn://svn.chromium.org/blink/trunk@156934 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Increase playForMillisecs timeout in video-played tests

Increases this timeout to reduce flakiness risk for the loop portion of the video-played-collapse layout test.
Uses window.performance.now() instead of 'new Date().getTime()' in video-played.js to be more resilient to time changes.

R=scherkus@chromium.org
BUG=280848
TEST=All video-played.js layout tests pass 100 runs each locally on debug linux while machine is heavily loaded.

Review URL: https://chromiumcodereview.appspot.com/23456007

git-svn-id: svn://svn.chromium.org/blink/trunk@156935 bbb929c8-8fbe-4397-9dbb-9b2b20218538