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This improves the adb_command_line tool as follows:

 * For WebView, throw an exception if someone tries to use this on a
   user device (WebView won't respect the flags anyway)
 * For Chrome, log a warning on user devices (Chrome is slightly more
   relaxed, but usually won't respect flags)
 * Minor refactoring to use already-existing helper methods to add some
   flags (no significant change to behavior here though).

R=agrieve@chromium.org

Bug: 918727
Test: Manual (try to write flags on user and userdebug emulators)
Change-Id: I78dc223bd882fc41d47d19bac6b2b2a26a6a53ac
Reviewed-on: https://chromium-review.googlesource.com/c/1458369
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Auto-Submit: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630131}