DevTools: Page.captureScreenshot() captures the wrong area of the window

BUG=302157
R=apavlov@chromium.org

Review URL: https://codereview.chromium.org/26245002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227274 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227274