AppCache: fix a browser crashing bug that can happen during updates.

      BUG=558589

Review URL: https://codereview.chromium.org/1463463003

Cr-Commit-Position: refs/heads/master@{#360967}
(cherry picked from commit e5c298b780737c53fa9aae44d6fef522931d88b0)

Review URL: https://codereview.chromium.org/1469253002 .

Cr-Commit-Position: refs/branch-heads/2526@{#468}
Cr-Branched-From: cb947c0153db0ec02a8abbcb3ca086d88bf6006f-refs/heads/master@{#352221}