Fork: Do not clear document background in Android WebView

Cherry-pick of uncommitted https://codereview.chromium.org/24676002/ PS2

Depends on Change-Id: Ica4cdeaa35b971b75ef1e664cd54f3fc7f95507a in
third_party/WebKit, which is also a fork from upstream Chromium, so this
is also a fork.

Conflicts:
	content/renderer/web_preferences.cc

BUG: 10750978

Change-Id: I8cab4b5ac209c55c2ac5a498cd4dc72f6e0a4229
5 files changed