Ensure LayerTreeHostImpl's current frame time is updated every frame

LayerTreeHostImpl's current frame time is currently updated after every
draw. This means that if, in some frame, we try (and fail) to activate
the pending tree but we don't draw, then when we eventually do draw we'll
be using an old current frame time. As a result, animations started during
that frame will have an old start time, and hence will appear to jump during
the following frame.

This CL makes us update the current frame time after every frame, regardless
of whether drawing occurred during the frame.

BUG=240820

Review URL: https://chromiumcodereview.appspot.com/15139007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@200920 0039d316-1c4b-4281-b951-d872f2087c98