TaskScheduler: Add stack frame with shutdown behavior.

The heuristic to generate magic signatures for shutdown hang is not
perfect, as shown in https://crbug.com/910463#c10. This results in
shutdown hangs being misattributed and taking longer to be fixed.
This CL adds a frame on the stack to allow correct attribution of
shutdown hangs caused by SKIP_ON_SHUTDOWN and BLOCK_SHUTDOWN tasks.

Bug: 910463
Change-Id: I9c5cad2712e99b0f6fd00a6f88775b787b17d26b
Reviewed-on: https://chromium-review.googlesource.com/c/1379819
Commit-Queue: François Doray <fdoray@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#617236}