Sync: add entity modification to FakeServer

This CL adds the ModifyEntity method to FakeServer. This method is used
to modify existing Sync data on the server-side in order to mimic the
commit of a different client. This CL also adds a desktop browser test,
DownloadModifiedBookmark, to make use of the feature.

Other changes in the CL:
1) Add two util methods to the Sync bookmarks test helper to count
   bookmarks by URL. This is needed because FakeServer::ModifyEntity
   does not currently handle top-level entity name changes.
2) Change the FakeServerEntity hierarchy so that its EntitySpecifics
   member is stored as a protected field in the base class. This is done
   so that the specifics can be modified as part of a FakeServer method.

BUG=365774

Review URL: https://codereview.chromium.org/1196023002

Cr-Commit-Position: refs/heads/master@{#336478}

0 files changed, 0 insertions, 0 deletions