cc: Adding a CommitEarlyOutReason enum to replace bool in BeginMainFrameAbort method.

The bool is confusing and has meant a bunch of tests have ended up not testing
what they thought they were testing. The new enum clearly delineates between
cases were we abort without handing the commit and when we abort because the
commit was an effective nop.

This also allows new early out reasons to be easily added.

Using a C++ "enum class" to make sure all BeginMainFrameAborted cases are
converted by preventing implicit conversion to bool. Allowed on;
https://chromium-cpp.appspot.com/

BUG=416749

Review URL: https://codereview.chromium.org/793243006

Cr-Commit-Position: refs/heads/master@{#308739}

• cc/BUILD.gn[diff]
• cc/cc.gyp[diff]
• cc/scheduler/commit_earlyout_reason.h[Added - diff]
• cc/scheduler/scheduler.cc[diff]
• cc/scheduler/scheduler.h[diff]
• cc/scheduler/scheduler_state_machine.cc[diff]
• cc/scheduler/scheduler_state_machine.h[diff]
• cc/scheduler/scheduler_state_machine_unittest.cc[diff]
• cc/test/layer_tree_test.cc[diff]
• cc/test/layer_tree_test.h[diff]
• cc/trees/layer_tree_host_impl.cc[diff]
• cc/trees/layer_tree_host_impl.h[diff]
• cc/trees/layer_tree_host_unittest.cc[diff]
• cc/trees/single_thread_proxy.cc[diff]
• cc/trees/single_thread_proxy.h[diff]
• cc/trees/thread_proxy.cc[diff]
• cc/trees/thread_proxy.h[diff]