Migrate ExtensionWebRequestApiTest::WebRequestURLFetcherInterception to SimpleURLLoader

CL replaces the uses of URLFetcher and URLFetcherDelegate by SimpleURLLoader in
ExtensionWebRequestApiTest::WebRequestURLFetcherInterception. Test is also renamed
to WebRequestURLLoaderInterception

Note that, it moves some local const char[] declarations up in the current method
body (WebRequestURLFetcherInterception), so that they can be used within the
Lambda functor.

BUG=773295

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: I552ee85f72be373a08aa7f36afe3ee83996cef6e
Reviewed-on: https://chromium-review.googlesource.com/1053889Reviewed-by: Dominic Battré <battre@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Cr-Commit-Position: refs/heads/master@{#558256}

testing/buildbot/filters/mojo.fyi.network_browser_tests.filter

@@ -143,7 +143,7 @@
 -ExtensionWebRequestApiTest.WebRequestTypes
 -ExtensionWebRequestApiTest.WebRequestTestOSDD
 # http://crbug.com/841002
--ExtensionWebRequestApiTest.WebRequestURLFetcherInterception
+-ExtensionWebRequestApiTest.WebRequestURLLoaderInterception
 -ExtensionWebRequestApiTest.WebRequestClientsGoogleComProtection
 # Note WebRequestUnloadImmediately is disabled on Linux
 -ExtensionWebRequestApiTest.WebRequestUnloadImmediately