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This is a follow up of [1]. Basically, WebCanvasCaptureHandler is now
only referenced within Blink, with its use is scattered between
renderer/modules/ and renderer/core/html/canvas.

For the of later, we can not simple replace the existing uses of
WebCanvasCaptureHandler in CanvasDrawListener by CanvasCaptureHandler
because renderer/core/html/canvas can not include renderer/modules/.

As per haraken's advice in [2], this CL makes CanvasDrawListener
pure virtual, and implements the methods as needed in
modules/mediacapturefromelement:

- AutoCanvasDrawListener: has the base implementation of the 3 method
  that used to live in html/canvas/CanvasDrawListener.

- OnRequestCanvasDrawListener: inherits from AutoCanvasDrawListener and
reimplements SendNewFrame.

- TimedCanvasDrawListener: inherits from OnRequestCanvasDrawListener
(but does not override any method).

This allows us not to duplicate any code among these 3 classes,
and eliminate WebCanvasCaptureHandler altogether.

[1] https://crrev.com/c/1600898 (Onion soup canvas_capture_handler.cc/h)
[2] https://crrev.com/c/1600898/1//COMMIT_MSG#34

BUG=787261,919392

Change-Id: I88113cc9cfe38a453ac028528f34eaea3448ea2e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1603722Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Cr-Commit-Position: refs/heads/master@{#658944}