[webgl] Handle trivial case in GLES2Implementation::ReadPixels

ReadPixels required the {pixels} parameter not to be nullptr, even when
it pointed into an ArrayBuffer of size 0. This requires the ArrayBuffer
implementation to allocate a backing store of size 0 instead of just
keeping a nullptr.

With this CL we handle the trivial case (width == 0 or height == 0) in
the beginning as a no-op, so that nullptr as backing store is okay.

Bug: chromium:1008840
Change-Id: If7337bf2bb625e4f8addb0d346c5db276b555695
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1893059Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712999}

third_party/blink/renderer/core/typed_arrays/array_buffer/array_buffer.h

@@ -139,7 +139,7 @@ scoped_refptr<ArrayBuffer> ArrayBuffer::Create(const void* source,
 }
 
 scoped_refptr<ArrayBuffer> ArrayBuffer::Create(ArrayBufferContents& contents) {
-  CHECK(contents.DataMaybeShared());
+  CHECK(contents.DataLength() == 0 || contents.DataMaybeShared());
   return base::AdoptRef(new ArrayBuffer(contents));
 }
 

third_party/blink/renderer/modules/webgl/webgl_rendering_context_base.cc

@@ -4446,8 +4446,21 @@ void WebGLRenderingContextBase::ReadPixelsHelper(GLint x,
     return;
   }
   ClearIfComposited();
+
   uint8_t* data = static_cast<uint8_t*>(pixels->BaseAddressMaybeShared()) +
                   offset_in_bytes.ValueOrDie();
+
+  // We add special handling here if the 'ArrayBufferView' is size '0' and the
+  // backing store is 'nullptr'. 'ReadPixels' creates an error if the provided
+  // data is 'nullptr'. However, in the case that we want to read zero pixels,
+  // we want to avoid this error. Therefore we provide temporary memory here if
+  // 'ArrayBufferView' does not provide a backing store but we actually read
+  // zero pixels.
+  base::Optional<Vector<uint8_t>> buffer;
+  if (!data && (width == 0 || height == 0)) {
+    buffer.emplace(32);
+    data = buffer->data();
+  }
   {
     ScopedDrawingBufferBinder binder(GetDrawingBuffer(), framebuffer);
     ContextGL()->ReadPixels(x, y, width, height, format, type, data);