Comment ListenToCookieChanges call in StoragePartitionImplMap::Get.

This adds a comment before the CookieStoreContext::ListenToCookieChanges
call, and renames one of its parameters to hopefully make it clearer.
If anyone else ever misreads the code the way I did, hopefully this
change will help explain things.

Change-Id: Id5256b85c179f9a16a2c5b0df18288bb3122e245
Reviewed-on: https://chromium-review.googlesource.com/c/1413890
Reviewed-by: Victor Costan <pwnall@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Robbie McElrath <rmcelrath@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623156}
3 files changed