Replace base::Callback with base::OnceCallback in base::PendingTask

After this CL, PendingTask holds the posted task as OnceClosure. And to
do that, PendingTasks get non-const on a task invocation, due to the
mutation the posted task for the consumption by the task invocation.

BUG=554299

Review-Url: https://codereview.chromium.org/2386653002
Cr-Original-Commit-Position: refs/heads/master@{#425318}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 739ffe3fd7b83bcb9ef7eb2e4b5c52fdbf35f59d
1 file changed