Fix overrides for Windows

Add not yet implemented overrides to render_widget_host_view_qt.h.
Remove ambiguous and unneeded override from browser_context_qt.h.

Change-Id: Id1cac9bf811d8e128e5ba8df059f7ca3bdd919dc
Reviewed-by: Michael Bruning <michael.bruning@digia.com>
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>
Reviewed-by: Andras Becsi <andras.becsi@digia.com>
Fix gpu::gles2::MailboxName on Windows

It is "struct GPU_EXPORT MailboxName" in
chromium/gpu/command_buffer/service/mailbox_manager.h.
MSVC won't accept class.

Change-Id: I979808f357b32e1d99fe70a1138bf292b54c4e76
Reviewed-by: Andras Becsi <andras.becsi@digia.com>
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>