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During navigation, in RenderFrameHostManager::CommitPending, when the
navigation results in creation of a new RenderWidgetHostView, the
sequence of events in that function is that
- the old RenderWidgetHostView is hidden
- the new RenderWidgetHostView takes content from the old one
- the new RenderWidgetHostView is shown

On macOS, this can result in a one-frame flicker where there is no
content present (between the hide and the show). Fix this by adding
a ScopedCocoaDisableScreenUpdates for the duration of CommitPending.
This will make all Cocoa changes in the function be atomic.

In order to include ScopedCocoaDisableScreenUpdates in a non-objective-C
file, move its ScopedCocoaDisableScreenUpdates implementation out of
the header file.

Bug: 829523
Change-Id: If541ef8985cf7990d63a79435b179f84339a9372
Reviewed-on: https://chromium-review.googlesource.com/1024968Reviewed-by: ccameron <ccameron@chromium.org>
Reviewed-by: Saman Sami <samans@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: ccameron <ccameron@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553628}