Remove Settings::is_shadow_page_ and its setter/getter

The referenced bug seems to be fixed, and nothing seems to read the
flag (anymore.)

Change-Id: I9975182364e9344e2dde88cb70211e78b10b69c1
Reviewed-on: https://chromium-review.googlesource.com/c/1314634
Reviewed-by: Luna Lu <loonybear@chromium.org>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#605222}
3 files changed