PlzWorker: Move browser-side worker classes to content/browser/worker_host/

This CL adds content/browser/worker_host/ directory, and moves files under
content/browser/dedicated_worker/ and content/browser/shared_worker/ to the
directory so that browser-side loader classes are sharable between dedicated
workers and shared workers.

See the design doc for details:
https://docs.google.com/document/d/1fWsD0oIa5sNDfUFWGJZ41pDo3zzsbFGyQSNdV8nOG4I/edit?usp=sharing

Bug: 906991
Change-Id: Ibe2a342495589abab10c08c83ab801ae4f1bceee
Reviewed-on: https://chromium-review.googlesource.com/c/1351198
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Christian Dullweber <dullweber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611503}
41 files changed