gpu: gpu decoder sets viewport size same to the size of the default framebuffer.

gpu/command_buffer/service/gles2_cmd_decoder.cc

Index: gpu/command_buffer/service/gles2_cmd_decoder.cc
diff --git a/gpu/command_buffer/service/gles2_cmd_decoder.cc b/gpu/command_buffer/service/gles2_cmd_decoder.cc
index 8c24d3cd1df5f8e04a458d173ec5106f5d3971bd..06842ec581a726beba1290d20ea93654c748e450 100644
--- a/gpu/command_buffer/service/gles2_cmd_decoder.cc
+++ b/gpu/command_buffer/service/gles2_cmd_decoder.cc
@@ -2681,6 +2681,10 @@ bool GLES2DecoderImpl::Initialize(
       return false;
     }
 
+    // |size| is set if the GL context is offscreen.
+    state_.viewport_width = size.width();
+    state_.viewport_height = size.height();
+
     // Allocate the offscreen saved color texture.
     DCHECK(offscreen_saved_color_format_);
     offscreen_saved_color_texture_->AllocateStorage(
@@ -2742,9 +2746,6 @@ bool GLES2DecoderImpl::Initialize(
     return false;
   }
 
-  state_.viewport_width = size.width();
-  state_.viewport_height = size.height();

[no sievers, 2015/03/02 20:37:12]

What's the bug?

We call InitState() in line 2758, which calls glViewport() with the value in
|state_|, so the driver's state will match our initial state.

Your patch changes the initial state for onscreen surface from 0,0 to 0,1 (w,h),
why is better?

If we want to match the spec better (viewport should match window size
initially) we could try surface_->GetSize() for onscreen contexts. But not sure
that will always work or fix anything. We probably rely on the client to do the
right thing.

[dshwang, 2015/03/03 09:20:29]

There's no bug. I just confused why onscreen viewport is initially 0, 0.
previous patch changes from 0,0 to 1,1
That's good point! onscreen viewport should match surface_->GetSize()
it's always work because surface must be created before this function.
It doesn't fix anything but it's what code is supposed to be.

-
   GLint viewport_params[4] = { 0 };
   glGetIntegerv(GL_MAX_VIEWPORT_DIMS, viewport_params);
   viewport_max_width_ = viewport_params[0];