Rename enum name in PrerenderManagerMode

Rename the name of the enum in PrerenderManagerMode.
The CL makes no functional changes.

Rename the variable to explicitly Mark the mode as DEPRECATED
since this mode is no longer supported.

Change-Id: I763aeaa3ec4b5c25fd7514c48d33ebd23e922dd1
Bug: 755921
TBR: rdevlin.cronin@chromium.org,fdoray@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1493300
Commit-Queue: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Egor Pasko <pasko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641998}
7 files changed