Fix ref leak when assigning Lengths with the same calc value

This patch adds a check to Length's assignment/copy constructor
to avoid taking an unnecessary ref of a calc object.

No regressions observed in the smoothness.tough_animation_cases
benchmark after this change.
Test command: tools/perf/run_benchmark --browser=android-content-shell --pageset-repeat=5 smoothness.tough_animation_cases
Device: Nexus 10

BUG=355303

Review URL: https://codereview.chromium.org/209843003

git-svn-id: svn://svn.chromium.org/blink/trunk@170739 bbb929c8-8fbe-4397-9dbb-9b2b20218538