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This extracts DevToolsNetworkInterceptor as an interface between
NetworkHandler and the implementation of network request interceptor.
The only current implementation is DevToolsURLRequestInterceptor, but
the URLLoader-based implementation will share the interface.

Bug: 812464
Change-Id: I3c3c00fce2bae7e9278b05eb688f3dda1e8b10c3
Reviewed-on: https://chromium-review.googlesource.com/920795
Commit-Queue: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537469}