Merge this with the internal method to generate a new command graph
node. Call onResourceChanged internally in beginWriteResource.

Bug: angleproject:2539
Change-Id: Ie33f886c5df7e15ff0b5d690a63fa664b1e964d4
Reviewed-on: https://chromium-review.googlesource.com/1069292
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Luc Ferron <lucferron@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>