base/threading: Lazy thread startup for pthreads
base/threading: Lazy thread startup for pthreads

r331235 made it possible to initialize MessageLoop lazily.
But, on platforms that depends on pthreads, thread creation
still blocks until the newly created thread has started.

This patch stop waiting for the new thread starts. This
got to be possible because I removed thread ID from
PlatformThreadHandle. (crbug.com/468793)

BUG=495097

Review URL: https://codereview.chromium.org/1150563002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#340746}

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