gtk: Simplify BrowserTitlebar::OnScroll() function.
gtk: Simplify BrowserTitlebar::OnScroll() function.

This patch greatly simplifies OnScroll() function by using the Browser accessors,
instead of going through tabstrip_model() (which causes a second indirection).

R=erg@chromium.org

Review URL: https://chromiumcodereview.appspot.com/10262003

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@135015 0039d316-1c4b-4281-b951-d872f2087c98

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