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TestSimpleTaskRunner::GetPendingTasks returns a full copy of the queued
pending tasks, which is less clean, and also that prevents a refactoring
that makes tasks move-only.

This CL replaces them with an equivalent NumPendingTasks to avoid the copy.

BUG=554299

Review-Url: https://codereview.chromium.org/2335153002
Cr-Commit-Position: refs/heads/master@{#418496}