cc: Use Scheduler::BeginFrame() in SingleThreadProxy

Proxy should use BeginFrame() to request new BeginFrame instead of BeginImplFrame().
The scheduler turns BeginFrames into BeginImplFrames or BeginRetroFrames
as necessary to make sure we don't start a new BeginImplFrame
before the previous deadline.

Some of public methods that are only used by itself moved into the private.

[email protected]
BUG=NONE
TEST=cc_unittests

Review URL: https://codereview.chromium.org/536123002

Cr-Commit-Position: refs/heads/master@{#293259}
diff --git a/cc/trees/single_thread_proxy.cc b/cc/trees/single_thread_proxy.cc
index acf481e3..1ccbdb9d 100644
--- a/cc/trees/single_thread_proxy.cc
+++ b/cc/trees/single_thread_proxy.cc
@@ -432,7 +432,7 @@
 void SingleThreadProxy::BeginFrame(const BeginFrameArgs& args) {
   TRACE_EVENT0("cc", "SingleThreadProxy::BeginFrame");
   if (scheduler_on_impl_thread_)
-    scheduler_on_impl_thread_->BeginImplFrame(args);
+    scheduler_on_impl_thread_->BeginFrame(args);
 }
 
 void SingleThreadProxy::CompositeImmediately(base::TimeTicks frame_begin_time) {