AppCache: Use WeakPtr<> to fix a potential uaf bug.

BUG=554908

Review URL: https://codereview.chromium.org/1441683004

Cr-Commit-Position: refs/heads/master@{#359930}
(cherry picked from commit 4b5eefe562bd45c059f5dd93079450db5610788a)

Review URL: https://codereview.chromium.org/1470183002 .

Cr-Commit-Position: refs/branch-heads/2564@{#94}
Cr-Branched-From: 1283eca15bd9f772387f75241576cde7bdec7f54-refs/heads/master@{#359700}
2 files changed