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When synchronizing page scale values between trees, setting the page scale
factor and deltas seperately means that the observer functions called when
the page scale factor was changed get called with a transient, incorrect
total page scale factor. This CL combines the seperate methods for updating
the page scale factor and page scale delta into one and updated tree
synchronizations to use this new one-shot function. The old methods remain
but call into the newly added method.

BUG=366249

Review URL: https://codereview.chromium.org/315003009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@276804 0039d316-1c4b-4281-b951-d872f2087c98