This adds a new NumberToUint8Clamped simplified operator that does the
round ties to even + clamping necessary to store to Uint8ClampedArrays.

BUG=v8:4470,v8:5267,v8:5615
R=jarin@chromium.org

Review-Url: https://codereview.chromium.org/2489563004
Cr-Commit-Position: refs/heads/master@{#40861}