ui: CHECK that UnPremultiply is passed a 32bpp image

To do otherwise results in accessing random data.

BUG=1147430
TEST=none

Change-Id: Icedacbaac64cad3fc903e6423c6f9aad8c1e8cb5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2531118
Commit-Queue: danakj <danakj@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826300}

ui/gfx/skbitmap_operations.cc

@@ -630,6 +630,8 @@ SkBitmap SkBitmapOperations::UnPreMultiply(const SkBitmap& bitmap) {
     return bitmap;
   if (bitmap.alphaType() != kPremul_SkAlphaType)
     return bitmap;
+  // It's expected this code is called with a 32bpp image.
+  CHECK_EQ(kN32_SkColorType, bitmap.colorType());
 
   const SkImageInfo& opaque_info =
       bitmap.info().makeAlphaType(kUnpremul_SkAlphaType);