[Blink] Remove check on makeImageSnapshot() from CanvasResourceSB

CanvasResourceSharedBitmap::UploadSoftwareRenderingResults() currently
checks whether the SkImage returned from SkSurface::makeImageSnapshot()
is null and early-outs if so. However, this check is redundant:

* Tracing through makeImageSnapshot()'s software rendering path shows
  that the cases in which it can return null boil down to the
  SkImageInfo being not valid [1][2][3]
* However, the SkSurface here comes from here [4], which will also
  return null if the SkImageInfo (which is the same) is not valid [5][6]
* In that case, the CreateSharedBitmapProvider() call would have failed
  in the first place [7][8]

[1] https://source.chromium.org/chromium/chromium/src/+/main:third_party/skia/src/image/SkImage_Raster.cpp;l=192-194;drc=32583061c525182327dd4bbeb3530fdd21e32d9c;bpv=1;bpt=1
[2] https://source.chromium.org/chromium/chromium/src/+/main:third_party/skia/src/image/SkImage_RasterFactories.cpp;l=167-169;drc=32583061c525182327dd4bbeb3530fdd21e32d9c
[3] https://source.chromium.org/chromium/chromium/src/+/main:third_party/skia/src/image/SkImage_RasterFactories.cpp;drc=32583061c525182327dd4bbeb3530fdd21e32d9c;l=33
[4] https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/platform/graphics/canvas_resource_provider.cc;l=229-235?q=canvas_resource_provider.cc&ss=chromium
[5] https://source.chromium.org/chromium/chromium/src/+/main:third_party/skia/src/image/SkSurface_Raster.cpp;l=187;drc=32583061c525182327dd4bbeb3530fdd21e32d9c;bpv=1;bpt=1
[6] https://source.chromium.org/chromium/chromium/src/+/main:third_party/skia/src/image/SkSurface_Raster.cpp;drc=32583061c525182327dd4bbeb3530fdd21e32d9c;l=36
[7] https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/platform/graphics/canvas_resource_provider.cc;l=1096-1103?q=CreateSharedBitmapProv&ss=chromium
[8] https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/platform/graphics/canvas_resource_provider.cc;l=246-248;drc=32583061c525182327dd4bbeb3530fdd21e32d9c?q=CreateSharedBitmapProv&ss=chromium

Bug: 379996128
Change-Id: I7d11a4e098211621570e4a8f09e0d2a2ef68ff61
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6216270
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1416669}