Reduce calls to LayoutUnit(int)

The integer constructor of LayoutUnit is relatively quite expensive;
the default constructor is a far cheaper way to initialize a LayoutUnit
with the value of 0.

Implicit conversions from int to LayoutUnit mean that tests against 0
are also surprisingly expensive.

This CL is a follow-on to r158483, which reduced CPU spent on the
LayoutUnit constructor from 6.x% to 3.x%; it further reduces CPU spent
in the function down below 1% on Nexus 7 for our heavily-repainting
test page.

BUG=299661
R=eae@chromium.org

Review URL: https://codereview.chromium.org/25021003

git-svn-id: svn://svn.chromium.org/blink/trunk@158571 bbb929c8-8fbe-4397-9dbb-9b2b20218538