Extensions: Prevent content script injection in the New tab Page.

r487664 disallowed content script injection in the New Tab Page. However, the
check in RendererPermissionsPolicyDelegate::IsRestrictedUrl for the same, might
not work due to the following reasons:
  - There might be a race between checking if the extension can inject the
    script and setting the new tab url in the renderer (SearchBouncer).
  - The New Tab page url in the SearchBouncer might be set incorrectly due to
    incorrect handling of multiple profiles by InstantService.

Fix this by checking if the current renderer process is an Instant (NTP)
renderer. This should work since the NTP renderer process should not be shared
with other sites.

BUG=844428, 662610

Change-Id: I45f6b27fb2680d3b8df6e1da223452ffee09b0d8
Reviewed-on: https://chromium-review.googlesource.com/1068607
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#563031}