Prevent bad casting in ImageBitmap when calling ArrayBuffer::createOrNull

Currently when ImageBitmap's constructor is invoked, we check whether
dstSize will overflow size_t or not. The problem comes when we call
ArrayBuffer::createOrNull some times in the code.

Both parameters of ArrayBuffer::createOrNull are unsigned. In ImageBitmap
when we call this method, the first parameter is usually width * height.
This could overflow unsigned even if it has been checked safe with size_t,
the reason is that unsigned is a 32-bit value on 64-bit systems, while
size_t is a 64-bit value.

This CL makes a change such that we check whether the dstSize will overflow
unsigned or not. In this case, we can guarantee that createOrNull will not have
any crash.

BUG=664139

Review-Url: https://codereview.chromium.org/2500493002
Cr-Commit-Position: refs/heads/master@{#431936}
ImageBitmap* ImageBitmap::create(PassRefPtr<StaticBitmapImage> image) {
  return new ImageBitmap(std::move(image));
}

ImageBitmap* ImageBitmap::create(PassRefPtr<StaticBitmapImage> image) {
  return new ImageBitmap(std::move(image));
}
