GitLab

Remove the following tab metrics because they're too uniform to be
useful in predictive models:

* ContentType: Almost always CONTENT_TYPE_TEXT_HTML (corresponding to
  normal web pages, as opposed to e.g. application/pdf).
* IsExtensionProtected: Almost never true.
* DefaultProtocolHandler: Rarely populated.

Bug: 784639
Change-Id: Ie40b268ab873d71305a59ae9b6356a8e105ceb31
Reviewed-on: https://chromium-review.googlesource.com/1036858Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: Michael Giuffrida <michaelpg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556000}