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This CL:
* uses ExtensionId as type instead of std::string
* adds empty default implementations to ExternalCacheDelegate since
  most delegates don't care about all On*() methods.

Bug: none
Change-Id: I37cce73d60708305b8746e20803216da21eefa9d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2366744
Commit-Queue: Alexander Hendrich <hendrich@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801323}