Even on a beefy workstation, resizing theme images takes a long time
(>1s). This causes a painful hiccup in the UI. This patch addresses that
by moving the operation to a helper thread. The actual application of
the theme won't be any faster but it won't block interaction.

As a future area of investigation it might be worth changing the "theme
installed" infobar to show earlier with a message like "installing
theme..." until this operation is completed, since otherwise it can feel
like nothing is happening.

original review: https://codereview.chromium.org/2799003002/
TBR=pkotwicz@chromium.org,pkasting@chromium.org,erg@chromium.org,pavely@chromium.org,rdevlin.cronin@chromium.org

Bug: 316070
Change-Id: I4e82bf144136cbaf3a3140ba39388bd8817e3ea9
Reviewed-on: https://chromium-review.googlesource.com/522912


Reviewed-by: Evan Stade <estade@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#476802}