Add an implict animation between ActionBar backgrounds

Add an implict animation between ActionBar backgrounds

This CL uses CSS transition to add an implicit animation between purple
ActionBar and the light grey "search" action bar. It's unclear to me how best
to create implicit animations in this system, but using a CSS transition works
surprisingly well.

Also, fix the typography style for the placeholder in the search box.
Previously it was white on Grey[50], which was extremely hard to see.

R=rafaelw@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/82033c3b8377fa31eabcb5151ddc862687a72d9d

[abarth-chromium, 2015-03-25 05:29:29]

abarth@chromium.org changed reviewers:
+ ojan@chromium.org, rafaelw@chromium.org

[rafaelw, 2015-03-25 17:04:15]

lgtm

[abarth-chromium, 2015-03-25 17:42:46]

Committed patchset #1 (id:1) manually as
82033c3b8377fa31eabcb5151ddc862687a72d9d (presubmit successful).