Adds notification when a viewmanagerclient is unembedded
Currently clients see OnViewDeleted(), which means the client has no
way to discern between a deletion and another app getting
embedded.
This is necessary so that html_viewer knows when it needs to swap what is a local frame to a remote frame. Currently html viewer sees the view as being destroyed and deletes the frame. If another frame is being embedded then deleting the frame isn't the right thing, instead the frame needs to be converted to remote. Adding OnUnembed() allows html viewer to detect the reuse case.
BUG=479172,490221
TEST=covered by tests
R=fsamuel@chromium.org
Review URL: https://codereview.chromium.org/
1276723003
Cr-Commit-Position: refs/heads/master@{#342004}