Add onBackendCreated callback to Viewer

Add onBackendCreated callback to Viewer

Also removes fWidth and fHeight from Window and instead
calls into WindowContent to get these values.

BUG=skia:

Change-Id: I72ee506004b7da73db9abb607a3bc82edfcf7d43
Reviewed-on: https://skia-review.googlesource.com/8795
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Yuqian Li <liyuqian@google.com>