Security fix: integer overflow on checking image size

Test is left in another CL (codereview.chromiu,.org/11274036) to avoid conflict there.  Hope it's fine.

BUG=160926


Review URL: https://chromiumcodereview.appspot.com/11410081

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@167882 0039d316-1c4b-4281-b951-d872f2087c98
skia::PlatformCanvas* PPB_ImageData_Impl::GetPlatformCanvas() {
  return backend_->GetPlatformCanvas();
}

skia::PlatformCanvas* PPB_ImageData_Impl::GetPlatformCanvas() {
  return backend_->GetPlatformCanvas();
}
