Use the same code to serialize app window state for the renderer

We send the properties of the app window to the renderer process at
window creation time and when the native app window state changes.
The code to serialize the state into a dictionary is duplicated
across two different locations. They should just use the same code.

BUG=None

Review URL: https://codereview.chromium.org/157133003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250023 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250023