customtabs: Add paranoia to isCallerForegroundOrSelf() due to NPEs in the wild.

We have a number of NPEs reported from devices in the
wild. Unfortunately, we only know that they happen on L pre-MR1, and
from warmup(). This commits adds null checks to prevent these from
crashing the client (reported by GSA).

BUG=654705

Review-Url: https://codereview.chromium.org/2404183004
Cr-Commit-Position: refs/heads/master@{#424406}
(cherry picked from commit f974fe4e5a32803b83592a014a82db683cb485f7)

Review URL: https://codereview.chromium.org/2415573002 .

Cr-Commit-Position: refs/branch-heads/2883@{#59}
Cr-Branched-From: 614d31daee2f61b0180df403a8ad43f20b9f6dd7-refs/heads/master@{#423768}
1 file changed