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In particular, replace about:net-internals with
chrome://net-export, add link to end-user instructions to
chrome://net-export itself, add a link to internal chrome network
crash page, and slightly update instructions.

Bug: None
Change-Id: I8ce872f1874ca632c7a1417fb9727fe4e4131778
Reviewed-on: https://chromium-review.googlesource.com/964943
Commit-Queue: Matt Menke <mmenke@chromium.org>
Reviewed-by: Eric Roman <eroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544402}