Remove XSS from chrome://downloads

BUG=668653
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2526323004
Cr-Commit-Position: refs/heads/master@{#435104}
(cherry picked from commit 6d9a7916b48581f72fda060a1210ebef7f89b229)

Review URL: https://codereview.chromium.org/2537893004 .

Cr-Commit-Position: refs/branch-heads/2924@{#218}
Cr-Branched-From: 3a87aecc31cd1ffe751dd72c04e5a96a1fc8108a-refs/heads/master@{#433059}
3 files changed