Store the omnibox start margin information in InstantService. When the omnibox bounds changes, InstantService notifies the observers about the margin change event. SearchTabHelper listens for the margin change event and sends a message to the underlying Instant page about the new start margin value.

As a prep to remove redundant Instant-related query params from search URLs, SearchTabHelper sends the margin information to the renderer process as soon as the navigation entry is committed.

BUG=247517
TEST=none

Review URL: https://codereview.chromium.org/148383012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249435 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249435