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This CL adds dark mode colors to the share extension. Besides colors,
it also removes the blur effect view. Before, there was a blur effect
over the background. There weren't any other views behind it though, so
the blur just helped color the background. Now, all the colors are
chrome-specific colors, so the background can be colored automatically.
Screenshots can be found in the linked bug.

Also, when building on iOS 13, the default presentation style for the
extension has changed to automatic, so the dimmed background is now
added automatically.

Finally, this modernizes some parts of share_extension_view.mm.

Bug: 992391
Change-Id: I16e5321ebc9018b41bfe617ad304fd3c3b6bb32e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1748754
Commit-Queue: Robbie Gibson <rkgibson@google.com>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#686358}