M35 Gardening: Forward WillReleaseScriptContext to the extension dispatcher

After r254810, WillReleaseScriptContext now arrives at the RenderFrameObserver,
so we need to create a helper observer just to catch this event.

Fixes XWalkExtensionsIFrameTest.KeepingReferenceToFunctionFromDestroyedIFrame.
Merge pull request #1781 from tmpsantos/next_extensions

M35 Gardening: Forward WillReleaseScriptContext to the extension dispatcher