1. Simplify top controls manager to never be in "overlay" mode, i.e. the top controls always push down the contents (note that the browser compositor is responsible for this; the renderer compositor just draws at the top of its surface and sends up the offset as metadata). 2. Introduce the concept of "visibleViewportSize". This is the true user-visible size on the current frame in DIP, not including top-controls or non-overlay scrollbars. This is used for max scroll bounds and scrollbar positioning. 3. Just disable clip layer masksToBounds if top controls are present, since it provides no value in the Android case. 4. Given the dynamism of the viewport size, abandon the WebKit scrollbar code and manually compute scrollbar quads after all. There are three inputs: trackRect (which is the layer size plus some margins); m_verticalAdjust which compensates for the fact the scrollbar layers are positioned by WebKit; and m_normalizedViewport which is a rect that encapsulates the position and size of both scrollbars. NOTRY=true BUG=161303 Review URL: https://chromiumcodereview.appspot.com/12519011 git-svn-id: svn://svn.chromium.org/chrome/trunk/src@188445 0039d316-1c4b-4281-b951-d872f2087c98