Update the QtWebEngineCore library to run on top of Chromium 37

Most of the patch is about upstream classes/methods that changed.

Other important details:
- icu data files are now used by default
- cygwin is no longer required to build on Windows
- RenderFrameHost has been replacing RenderViewHost in a few places,
  following the separate process iframes support in Chromium
- The user agent is accessed through ContentClient::GetUserAgent
  instead of from the command line switches

Change-Id: I86cc93aff7ce31176a80b0b4a5d54025674a451c
Reviewed-by: Andras Becsi <andras.becsi@digia.com>