Avoid using stale UserScript pointers

BUG=625393
TEST=Manually. Compile Chrome with ASAN and follow the steps from the bug report.

Review-Url: https://codereview.chromium.org/2116923002
Cr-Commit-Position: refs/heads/master@{#403725}
(cherry picked from commit a80776332fd8c99b58beab5d91a6675e85013628)

Review URL: https://codereview.chromium.org/2158683002 .

Cr-Commit-Position: refs/branch-heads/2743@{#656}
Cr-Branched-From: 2b3ae3b8090361f8af5a611712fc1a5ab2de53cb-refs/heads/master@{#394939}
1 file changed