cc: Adding BeginFrameTracker object and removing Now() from LTHI.

**Sheriffs**: If this patch is making a test flaky / fail,
then the test was already broken and this just makes the
problem visible.

This patch reduces the jitter in LTHI's animation time to
be identical to the vsync jitter rather than being affected
by the system load and OS scheduling delays.

BeginFrameTracker uses DCHECKs and TRACE_EVENT to strictly
track how the BeginFrameArgs are used throughout the LTHI.
Using this information incorrect usage of Now() has been
eliminated. Methods which violate correct usage of
BeginFrameArgs are now clearly marked to prevent
regressions.

BUG=346230

Review URL: https://codereview.chromium.org/787763006

Cr-Commit-Position: refs/heads/master@{#328911}

• cc/BUILD.gn[diff]
• cc/cc.gyp[diff]
• cc/scheduler/begin_frame_tracker.cc[Added - diff]
• cc/scheduler/begin_frame_tracker.h[Added - diff]
• cc/scheduler/scheduler.cc[diff]
• cc/scheduler/scheduler.h[diff]
• cc/scheduler/scheduler_unittest.cc[diff]
• cc/test/fake_layer_tree_host_impl.cc[diff]
• cc/test/fake_layer_tree_host_impl.h[diff]
• cc/test/scheduler_test_common.h[diff]
• cc/trees/layer_tree_host_impl.cc[diff]
• cc/trees/layer_tree_host_impl.h[diff]
• cc/trees/layer_tree_host_impl_unittest.cc[diff]
• cc/trees/layer_tree_host_unittest_context.cc[diff]
• cc/trees/layer_tree_impl.cc[diff]
• cc/trees/layer_tree_impl.h[diff]