Remove Dangerous indicator after going back from interstitial

While showing a Safe Browsing interstitial, the top-level hostname for
the WebContents gets added to a "pending" whitelist set. Previously, we
were never removing URLs from that set, meaning that if you went Back
from an interstitial to a URL on the same hostname, the Dangerous
indicator persisted.

This CL fixes this bug by removing pending whitelist URLs when an
interstitial is dismissed with the Don't Proceed option.

BUG=659709
TEST=Visit http://testsafebrowsing.appspot.com, click first link. Observe that the icon in the omnibox is a red triangle. Click "Back to safety". Observe that the icon in the omnibox is an (i), not a red triangle.
TEST=Added browser test

Review-Url: https://codereview.chromium.org/2451623005
Cr-Commit-Position: refs/heads/master@{#428932}
6 files changed