Enable GC annotations by default.

About:tracing needs compiled-in GC annotations support. See discussion here: https://codereview.chromium.org/15967010/

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

Review URL: https://chromiumcodereview.appspot.com/18348009

git-svn-id: svn://svn.chromium.org/blink/trunk@153435 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Populate default content in drag clipboard before dragstart event.

According to the spec [1], the drag data store should be populated
before the dragstart event is dispatched. Firefox implements this
behavior as well.

[1] http://www.whatwg.org/html/dnd.html#drag-and-drop-processing-model

BUG=251796
R=tony@chromium.org

Review URL: https://codereview.chromium.org/18174011

git-svn-id: svn://svn.chromium.org/blink/trunk@153436 bbb929c8-8fbe-4397-9dbb-9b2b20218538