chromeos: Show onscreen bubble when brightness changes.

This adds a BrightnessLibrary class that registers to receive
notification of brightness changes from the libcros code
that I added in http://codereview.chromium.org/5640003/,
a BrightnessBubble class to control an onscreen indicator of
the current brightness level, and a BrightnessObserver class
that wires up the two.

This change depends on
http://codereview.chromium.org/5603012/, which pulls the
updated version of libcros into Chrome.

BUG=chromium-os:8473,chromium-os:4816
TEST=built and tried it

Review URL: http://codereview.chromium.org/5620004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@68879 0039d316-1c4b-4281-b951-d872f2087c98
15 files changed