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Similar to http://crrev/c/1266611, this dumps WebView's CommandLine
state (including the "--enable-features" and "--disable-features"
switches) to the log when the "dummy app"
("org.chromium.webview_log_verbosifier") is present.

Notably, this will give us an idea of whether the NetworkService is
enabled or not (FieldTrialList + CommandLine state + knowledge of
whether it is enabled-or-disabled-by-default should be sufficient).

This also adds a usage guide for the webview verbosifier app, including
the new commandline stuff.

Skipping presubmit for LOG(INFO).

NOPRESUBMIT=true

Bug: 929886, 918221
Test: Manual (following the steps in the new docs)
Test: Upload to gerrit > open file > click "gitiles"
Change-Id: I4c698d661c340f760e2a90da20439b44a56ea38b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1573087
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Reviewed-by: Paul Miller <paulmiller@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652642}