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- Large numbers of redundant requests are degrading the performance
  of the SMB FSP
- This CL short circuits those requests while separately we can try
  to eliminate their root cause
- This filters out 2 types of requests
  1) Empty requests. These requests should never even be made since
     they are requests that explicitly ask for no information.
  2) Thumbnail only requests. We never have thumbnail data and always
     provide a hardcoded response.
- By short circuiting them here we can prevent them being sent to
  the service and incuring any network operations.

BUG=chromium:867200,chromium:587231
TEST=build and test

Change-Id: Ibb9eaaa175660106dd6a169781cc2ce4a83dafea
Reviewed-on: https://chromium-review.googlesource.com/1156989Reviewed-by: May Lippert <maybelle@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Commit-Queue: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579516}