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Currently, invalidate_shadow_on_frame_swap_ is not set when a MacViews window is
shown. It is only set when the layer is created, changed or the window resized.
This causes translucent windows like the "Restore Pages" bubble and Find-In-Page
window to not have a shadow, when they are shown after being hidden.

This CL modifies BridgedNativeWidget::OnVisibilityChanged() to trigger shadow
invalidation while showing translucent windows. This fixes the shadow for the
"Restore Pages" bubble and also of the Find-In-Page window on tab switching. A
unit test is also added which demonstrates the problem.

Note that this does not fix the shadow for the Find-In-Page window when it is
displayed initially on pressing Cmd+F.

BUG=636707, 646734

Review-Url: https://codereview.chromium.org/2336983002
Cr-Commit-Position: refs/heads/master@{#418803}