DevTools protocol interception, blocking & modification of requests

Adds Network.enableFetchInterception which sends Network.interceptedRequest for fetch requests and Network.interceptedRedirect for any redirects.  The fetch or redirect
is paused until one of the following commands is sent: Network.allowRequest, Network.blockRequest, Network.mockResponse, Network.modifyRequest.

Design doc: https://docs.google.com/document/d/1nEN0SqC1jEpHgKuQPrppwSabC8JRAuJb7MSTGCe2Pzg/edit#

BUG=702384, 546953
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_site_isolation

Review-Url: https://codereview.chromium.org/2739323003
Cr-Commit-Position: refs/heads/master@{#476635}
49 files changed