Merge 54767 - [Mac] M6 Omnibox positioning changes....

Merge 54767 - [Mac] M6 Omnibox positioning changes.

3px from top and bottom of images to cell edges in popup.  With this
change baselines worked out without tweaking.

Omnibox popup text wasn't quite aligned with field because previous
change used leading 'w' (regression).  This change measured using
leading 'P'.

Adjust keyword-hint decoration spacing around the image.  Also adjust
baseline to match font (regression).

Modify ImageDecoration to center the image horizontally, which allows
the decoration width to be independent of the image width.
PageActionDecoration uses this to pin the effective size at
|kPageActionIconMaxSize|.

Position page-action and bookmark popups so top border is at same gap
from Omnibox (2px) as Omnibox popup.  The mocks actually show the
bookmark point as an additional px from the star "crotch", but I think
this is reasonable for M6.

BUG=50575, 50765
TEST=none.  Everything still works?

Review URL: http://codereview.chromium.org/3015046

TBR=shess@chromium.org

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=54838