gpu: Disallow zero sized offscreen framebuffers

Modify initial the offscreen size if empty
DCHECK to assert that we don't run into this situation again

BUG=481035

Review URL: https://codereview.chromium.org/1106773002

Cr-Commit-Position: refs/heads/master@{#327180}

content/common/gpu/gpu_command_buffer_stub.cc

@@ -201,6 +201,14 @@ GpuCommandBufferStub::GpuCommandBufferStub(
 
   use_virtualized_gl_context_ |=
       context_group_->feature_info()->workarounds().use_virtualized_gl_contexts;
+
+  bool is_offscreen = surface_id_ == 0;
+  if (is_offscreen && initial_size_.IsEmpty()) {
+    // If we're an offscreen surface with zero width and/or height, set to a
+    // non-zero size so that we have a complete framebuffer for operations like
+    // glClear.
+    initial_size_ = gfx::Size(1, 1);
+  }
 }
 
 GpuCommandBufferStub::~GpuCommandBufferStub() {

gpu/command_buffer/service/gles2_cmd_decoder.cc

@@ -2464,6 +2464,7 @@ bool GLES2DecoderImpl::Initialize(
   TRACE_EVENT0("gpu", "GLES2DecoderImpl::Initialize");
   DCHECK(context->IsCurrent(surface.get()));
   DCHECK(!context_.get());
+  DCHECK(!offscreen || !offscreen_size.IsEmpty());
 
   ContextCreationAttribHelper attrib_parser;
   if (!attrib_parser.Parse(attribs))