cc: Remove BeginFrameSourcesConstructor.

Customizing begin frame sources is easier using a static Create method
instead of using a special class for that. Also get rid of
VSyncParameterObserver as a result.

R=brianderson,mithro
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1201573002

Cr-Commit-Position: refs/heads/master@{#335605}

• cc/BUILD.gn[diff]
• cc/cc.gyp[diff]
• cc/output/vsync_parameter_observer.h[Deleted - diff]
• cc/scheduler/begin_frame_source.cc[diff]
• cc/scheduler/begin_frame_source.h[diff]
• cc/scheduler/scheduler.cc[diff]
• cc/scheduler/scheduler.h[diff]
• cc/scheduler/scheduler_unittest.cc[diff]
• cc/surfaces/onscreen_display_client.cc[diff]
• cc/surfaces/onscreen_display_client.h[diff]
• cc/test/scheduler_test_common.cc[diff]
• cc/test/scheduler_test_common.h[diff]
• cc/trees/single_thread_proxy.cc[diff]
• cc/trees/single_thread_proxy.h[diff]
• cc/trees/thread_proxy.cc[diff]