Use ExceptionState::indexExceedsMinimumBound() to throw DOMException for SourceBuffer.

BUG=

Review URL: https://codereview.chromium.org/301653002

git-svn-id: svn://svn.chromium.org/blink/trunk@175331 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Mark the drag-and-drop and context-menu leaks as fixed.

https://src.chromium.org/viewvc/blink?revision=175257&view=revision
looks like fixed the drag-and-drop leak (crbug.com/364392).

The context-menu leak (crbug.com/364395) is not observed at this time.

BUG=364392,364395
NOTRY=True

Review URL: https://codereview.chromium.org/306053012

git-svn-id: svn://svn.chromium.org/blink/trunk@175332 bbb929c8-8fbe-4397-9dbb-9b2b20218538