Simplify BackgroundImageGeometry by removing meaningless distinctions.

m_destOrigin is only set at the very end of
BoxPainter::calculateBackgroundImageGeometry, and then always to
destRect().location(). m_destRect is not changed after this point.

m_destOrigin is only read in relativePhase(), which is only called after the
end of calculateBackgroundGeometry.

As a result, relativePhase() always returns m_phase, and we should be able to
safely remove both relativePhase and destOrigin. It's not entirely clear to me
what these were supposed to do, but at the moment they do nothing.

TEST=LayoutTests

Review URL: https://codereview.chromium.org/660523003

git-svn-id: svn://svn.chromium.org/blink/trunk@183771 bbb929c8-8fbe-4397-9dbb-9b2b20218538
GC::drawConvexPolygon() & friends cleanup

Assorted tweaks:

* Both drawConvexPolygon() clients (ObjectPainter::drawSolidBoxSide
  & RenderThemeChromiumMac::paintMenuListButton) turn off stroking,
  so we're always in fill-only mode. Update the method to reflect
  this limited scope, and stop mutating the global stroke style.
* Take an explicit color param to avoid changing (and leaking in
  the case of drawSolidBoxSide) the global fill color.

(because of the above, drawSolidBoxSides no longer dirties the GC paint
state in the common case => reduced pressure for committing deferred
paint saves).

* All drawConvexPolygon & clipConvexPolygon callers except
  RenderThemeChromiumMac pass in four points (quads), and per comments
  in setPathFromConvexPoints we don't assume convexity in this common
  case => update the method names to reflect this reality.

R=schenney@chromium.org,senorblanco@chromium.org

Review URL: https://codereview.chromium.org/657023006

git-svn-id: svn://svn.chromium.org/blink/trunk@183772 bbb929c8-8fbe-4397-9dbb-9b2b20218538