Fix possible map::end() dereference in AppCacheUpdateJob triggered by a compromised renderer.

BUG=551044

Review URL: https://codereview.chromium.org/1418783005

Cr-Commit-Position: refs/heads/master@{#358815}
(cherry picked from commit 2cb006bc9d3ad16353ed49c2b75faea618156d0f)

Review URL: https://codereview.chromium.org/1472943003 .

Cr-Commit-Position: refs/branch-heads/2526@{#466}
Cr-Branched-From: cb947c0153db0ec02a8abbcb3ca086d88bf6006f-refs/heads/master@{#352221}
5 files changed