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This padding was in some previous versions but got lost somewhere along
the way. This change restores it so there are always at least 12 pixels
between the baseline of the last line of text and the bottom of the card
or the image below the text. This will be true for WebKit, simple,
basic, and image notifications but also for list ones.

Note that although there will be exactly 12 pixels of space on Windows,
there will be 13 pixels On Chrome OS because different fonts are used
there, and also that if the Chrome OS system font changes that number of
pixels may change slightly.

This change also makes the padding between the top of notification
text and the notification card be 12 pixels, as it was designed to be.
It had somehow become 11 pixels.

BUG=225871
TEST=Bring up a notification, take a screenshot, and count pixels.

Review URL: https://chromiumcodereview.appspot.com/13560002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@192957 0039d316-1c4b-4281-b951-d872f2087c98