[Script Injection] Allow whitelisted extensions to inject scripts everywhere
[Script Injection] Allow whitelisted extensions to inject scripts everywhere

Fix a regression where whitelisted extensions couldn't inject content scripts on
other extension pages.

BUG=517370
TBR=kalman@chromium.org

Review URL: https://codereview.chromium.org/1294393003

Cr-Commit-Position: refs/heads/master@{#345382}
(cherry picked from commit 29d2ec93b685a04c1ad0ad047ab702c3cc62ba3f)

Review URL: https://codereview.chromium.org/1305223007 .

Cr-Commit-Position: 62bea078bfe806fd4d8dc2d3869914b6c8093498@{#448}
Cr-Branched-From: ef8f355264e75087b06072b03d328c50bf72843a@{#338390}

hgl888/chromium-crosswalk,hgl888/chromium-crosswalk,hgl888/chromium-crosswalk,hgl888/chromium-crosswalk,hgl888/chromium-crosswalk,hgl888/chromium-crosswalk,hgl888/chromium-crosswalk,hgl888/chromium-crosswalk,hgl888/chromium-crosswalk