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Instead of directly creating and binding pp::Graphics2D in PaintManager,
delegate these operations to PaintManager::Client. This provides several
benefits:

1. Allows the PaintManager::Client to return a different pp::Graphics2D
   implementation in the future. This will be useful especially once we
   replace pp::Graphics2D with a Pepper-free API.
2. Eliminates the need for PaintManager to interact with pp::Instance.

Bug: 1099020
Change-Id: I46ce18cfb3ced088afdf6581fc335aa56b8be195
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2303989
Commit-Queue: K. Moon <kmoon@chromium.org>
Reviewed-by: Daniel Hosseinian <dhoss@chromium.org>
Reviewed-by: Henrique Nakashima <hnakashima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789565}