Use fast default constructors for containers.

LayoutUnit() is fast; LayoutUnit(int) is slow because of bounds-checks.
Since the default constructor sets the m_value of the LayoutUnit to 0,
we can replace calls to LayoutUnit(0) with the default constructor.

Doing this for just these two files yields 5% speedup on the Blink
Layout performance test layers_overlap_2d.html.  It isn't just an
artifact of our artificial performance tests; we see LayoutUnit(int)
as 6% of a CPU profile on both Android and Linux for the page
http://allegro.pl/listing/listing.php?string=nexus+4.

BUG=299661
R=eae@chromium.org

Review URL: https://chromiumcodereview.appspot.com/24997004

git-svn-id: svn://svn.chromium.org/blink/trunk@158483 bbb929c8-8fbe-4397-9dbb-9b2b20218538