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Currently, it's controlled by switches, which can be used for
global control of the feature, but don't work well for per-tab
configuration.

Move the decision to ContentRendererClient and use the switches
in default implementation to maintain existing default behaviour.

Bug: 862698
Change-Id: I917e21ec8d8e493eef53113fe667d45ab17a48b5
Reviewed-on: https://chromium-review.googlesource.com/1133975
Commit-Queue: Luke Halliwell <halliwell@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577759}