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This CL fixes two bugs in the page restoration placeholder:

1) The alpha of the placeholder was not previously being reset to 1.0.

2) The page restoration placeholder was only being shown at the start
of a navigation. This means that the user will open a tab, it will
momentarily show blank (since the page has not yet been restored)
before showing the black/white placeholder. Now the black/white page
will show immediately as the user enters the tab.

Bug: 927843
Change-Id: If8ad4c60e05c84a240182316484e2657d46efaf0
Reviewed-on: https://chromium-review.googlesource.com/c/1450438
Commit-Queue: edchin <edchin@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628579}