Avoid unnecessary float<->LayoutUnit conversions in RenderVTTCue

RenderVTTCue::m_fallbackPosition is only used to store the initially
computed position of the cue - it does not need to be a FloatPoint.
Change the type to LayoutPoint and rename to m_defaultPosition, since
that better reflects the naming in spec text.
Also cleanup the includes a bit while here.

BUG=350474

Review URL: https://codereview.chromium.org/847813003

git-svn-id: svn://svn.chromium.org/blink/trunk@188273 bbb929c8-8fbe-4397-9dbb-9b2b20218538
2 files changed