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Add a code path to have vulkan webview draw into an intermediate buffer
and draw the buffer into the secondary command buffer.

Add a Feature to turn it on globally, which is only helpful for
debugging any potential problems from vulkan composite. Dynamically
switching between direct compopsite and intermediate buffer requires
more plumbing.

Bug: 919868
Change-Id: I059087432f3130372b795da6588069aa44dd4d9a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2605725Reviewed-by: Peng Huang <penghuang@chromium.org>
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Commit-Queue: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840850}