Re-land alexbost's experimental Offscreen Tabs API work.

Re-land alexbost's experimental Offscreen Tabs API work.

Original code review was http://codereview.chromium.org/7720002/ .

Rebased to top of tree. Fixed issues with custom bindings not getting called.
Reimplemented custom bindings to copy objects rather than deleting properties
of incoming event objects.

Eliminated duplicate constants. Refactored majority of
ToDataUrlOffscreenTabFunction and UpdateOffscreenTabFunction. Refactored code
copying modifiers from JS events to WebInputEvents.

BUG=110833
TEST=OffscreenTabsApiTest_OffscreenTabs