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We introduced a mistake where if the image was not opaque, the
background color was no longer drawn behind it, so on chromeos the
chrome header ui could be seen through. This fixes the issue by
rearranging the painting some, and improving some performance corner
cases while we're here.

First, we only need to use SaveLayerWithFlags() when we are blending
together 2+ things *that are not opaque*, then want to apply alpha to
them together. This is only needed when alpha is < 255, or when we have
both |frame_image| and |frame_overlay_image|. Otherwise the background
color is opaque, so drawing it first then blending one image into it is
equivalent.

Then, in both this and the other case, we draw the background color
first, then draw the images above it, in order to ensure the bar is
opaque.

Note also that we use kPlus blending mode when alpha < 255 so that
between the active and inactive background colors, they add up to
255 alpha, giving us something opaque.

R=estade@chromium.org

Bug: 836023
Change-Id: Ibc6b7a3dbec0d9d63b0b07f70a4c17405a3eeec2
Reviewed-on: https://chromium-review.googlesource.com/1050690Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Commit-Queue: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557265}