[chromium] Make WebLayerTreeView::loseCompositorContext non-pure virtual
https://bugs.webkit.org/show_bug.cgi?id=105328

Reviewed by James Robinson.

This allows us to decouple the chromium and webkit sides and remove this
function entirely on each side.

* chromium/public/WebLayerTreeView.h:
(WebKit::WebLayerTreeView::loseCompositorContext):


git-svn-id: https://svn.webkit.org/repository/webkit/trunk/Source/Platform/chromium/public@138052 268f45cc-cd09-0410-ab3c-d52691b4dbfc