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Appears calling the syscall over and over can be slow.

Use TLS to stash the result of the syscall.

Benchmarks:

Without change:
taskset -c 0 base_perftests --gtest_filter=SequenceManagerPerfTest.RunTenThousandImmediateTasks_OneQueue*
*RESULT task: run 10000 immediate tasks with one queue SequenceManager with message loop= 23483.009389671362 us/run
*RESULT task: run 10000 immediate tasks with one queue SequenceManager with message pump= 17142.489726027397 us/run
*RESULT task: run 10000 immediate tasks with one queue message loop= 5126.6598360655735 us/run
*RESULT task: run 10000 immediate tasks with one queue single thread in WorkerPool= 9817.776470588235 us/run

With change:
taskset -c 0 base_perftests --gtest_filter=SequenceManagerPerfTest.RunTenThousandImmediateTasks_OneQueue*
*RESULT task: run 10000 immediate tasks with one queue SequenceManager with message loop= 15673.59375 us/run
*RESULT task: run 10000 immediate tasks with one queue SequenceManager with message pump= 8832.218694885361 us/run
*RESULT task: run 10000 immediate tasks with one queue message loop= 5149.919670442842 us/run
*RESULT task: run 10000 immediate tasks with one queue single thread in WorkerPool= 9926.059523809523 us/run

BUG=898294

Change-Id: I414ff18fd0c92a3837b50e20165bf295b4f63082
Reviewed-on: https://chromium-review.googlesource.com/c/1291710Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602430}