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Before this CL
WebUIDataSource::SetRequestFilter() accepted a single callback function as a parameter.
That callback was responsible for
 - communicating via a boolean whether a given URL will be handled.
 - producing the response (bytes) for this request.

This is inflexible, because there are cases where we need to know whether a URL
will be handled by the request filter, without triggering the request yet. This is in
preparation of simplifying the mechanism that WebUIDataSourceImpl uses to determine
when a WebUI resource is gzipped.

After this CL:
WebUIDataSource::SetRequestFilter() accepts two callback functions.
 - |should_handle_request_callback| simply returns a boolean, indicating whether the
   request will be handled by the request filter, without handling the request.
 - |handle_request_callback| handles the request, without returning any boolean.

TBR=jam@chromium.org

Bug: 738243
Change-Id: I915b87801908885bcbf05e785bbacf8ad140c481
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1535164
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: calamity <calamity@chromium.org>
Reviewed-by: Dan Beam <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649289}