refactor drawing to support layers

Drawing elements that appear atop or below layers need to be
drawn both in the proper order and with the correct canvas to
respect clipping and the matrix.

The logic of what to draw is still in WebView.java, but the
actual drawing calls are now triggered inside the layer code.

This still draws layers incorrectly after drawing the history;
will fix this in a future CL.

Move drawing to WebView.cpp.
Use inverseScale to simplify viewPort metrics.
Remove root layer; let WebView.cpp handle it exclusively.

Requires companion fix in external/webkit.

http://b/2457316
http://b/2454127
http://b/2454149