Minimize performance regression caused by r157508

Minimize performance regression caused by r157508:
- Cache the value returned by Element::attributeItem() as it hides
  an if branch in ElementData::attributeBase().
- Minimize the cost of calling attrIfExists() which does a
  linear search by using iterators (thus avoiding the release
  assertion in Vector::at())

I get a 3.1% improvement locally on Bindings/id-setter.html 
performance test.

R=haraken
BUG=293663

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=161383

Review URL: https://codereview.chromium.org/60143002

git-svn-id: svn://svn.chromium.org/blink/trunk@161419 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Remove duplicated rule from core.gyp. This rule is covered by config.gyp.

Review URL: https://codereview.chromium.org/60633007

git-svn-id: svn://svn.chromium.org/blink/trunk@161420 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Make FrameTestHelpers::runPendingTasks wait for Timers.

ThreadTimers::sharedTimerFiredInternal() only runs Timers for 50ms before
yielding to the MessageLoop.  On heavily-loaded bots, this meant that
runPendingTasks wasn't enough to wait for components that use 0-delay Timers to
yield to the MessageLoop.

This deflakes WebFrameCSSCallbackTest: CSSSelectorWatch uses a 0-delay Timer,
and the test wants to wait for it to complete.

BUG=306952

Review URL: https://codereview.chromium.org/59613008

git-svn-id: svn://svn.chromium.org/blink/trunk@161421 bbb929c8-8fbe-4397-9dbb-9b2b20218538
IDL compiler: [Custom] for methods

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/61413003

git-svn-id: svn://svn.chromium.org/blink/trunk@161422 bbb929c8-8fbe-4397-9dbb-9b2b20218538
DevTools: Add support for adding folders to workspace with drag and drop (renderer part)

browser part: https://chromiumcodereview.appspot.com/57073004/

BUG=314476
R=pfeldman, kinuko

Review URL: https://codereview.chromium.org/57373002

git-svn-id: svn://svn.chromium.org/blink/trunk@161423 bbb929c8-8fbe-4397-9dbb-9b2b20218538