Do not clear canvas with transparent color after creating SkSurface

Recently SkSurface::MakeRaster changed its implementation to zero its
malloced space. So after creating the SkSurface, there is no need to
call sksurface's canvas->clear(SK_ColorTRANSPARENT). This CL removes
that step.

Review-Url: https://codereview.chromium.org/2119623003
Cr-Commit-Position: refs/heads/master@{#403907}
3 files changed