Cherry-pick: Do not clear root frame buffer for android webview

Cherry-pick of unlanded https://codereview.chromium.org/210443004/

Original description:

Android webview does not own the surface that compositor is drawing
into, and so should in general not it before draw.

BUG: 13134274
Change-Id: I90e8f39bd455f2494dbdcf48a40f15f837cf76fb
1 file changed