Do not call glDrawBuffersARB when the extension is absent

The function Framebuffer::ChangeDrawBuffersHelper was assuming
that binding of an FBO with no GL_COLOR_ATTACHMENT0 was
accompanied by setting the first draw buffer to GL_NONE with
glDrawBuffersARB in the fix introduced in:
https://codereview.chromium.org/315283002

When the extension is not present, it can be assumed that the
glDrawBuffersARB state need not be set and restored.

BUG=(wrangling)
NOTRY=True

Review URL: https://codereview.chromium.org/375413003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@282393 0039d316-1c4b-4281-b951-d872f2087c98

gpu/command_buffer/service/framebuffer_manager.cc

@@ -374,6 +374,11 @@ bool Framebuffer::HasUnclearedColorAttachments() const {
 }
 
 void Framebuffer::ChangeDrawBuffersHelper(bool recover) const {
+  // There will always be only one buffer, GL_COLOR_ATTACHMENT0, if
+  // GL_ARB_draw_buffers is not present, even if this FBO has no attachment.
+  if (!gfx::g_driver_gl.ext.b_GL_ARB_draw_buffers)
+    return;
+
   scoped_ptr<GLenum[]> buffers(new GLenum[manager_->max_draw_buffers_]);
   for (uint32 i = 0; i < manager_->max_draw_buffers_; ++i)
     buffers[i] = GL_NONE;

gpu/command_buffer/service/gles2_cmd_decoder.cc

@@ -3122,6 +3122,7 @@ bool GLES2DecoderImpl::CheckFramebufferValid(
       bool reset_draw_buffer = false;
       if ((backbuffer_needs_clear_bits_ | GL_COLOR_BUFFER_BIT) != 0 &&
           group_->draw_buffer() == GL_NONE) {
+        DCHECK(gfx::g_driver_gl.ext.b_GL_ARB_draw_buffers);
         reset_draw_buffer = true;
         GLenum buf = GL_BACK;
         if (GetBackbufferServiceId() != 0)  // emulated backbuffer