Framebuffer clear() needs to consider the situation some draw buffers are disabled.

gpu/command_buffer/service/framebuffer_manager.cc

Index: gpu/command_buffer/service/framebuffer_manager.cc
diff --git a/gpu/command_buffer/service/framebuffer_manager.cc b/gpu/command_buffer/service/framebuffer_manager.cc
index 7dfba400d24704d31a79cba36c62655b1c81bba8..4022b3defaa1e286e85e72f7ac6aa702fa1e9fb2 100644
--- a/gpu/command_buffer/service/framebuffer_manager.cc
+++ b/gpu/command_buffer/service/framebuffer_manager.cc
@@ -360,6 +360,55 @@ bool Framebuffer::HasUnclearedAttachment(
   return false;
 }
 
+bool Framebuffer::HasUnclearedColorAttachments() const {
+  for (AttachmentMap::const_iterator it = attachments_.begin();
+       it != attachments_.end(); ++it) {
+    if (it->first >= GL_COLOR_ATTACHMENT0 &&
+        it->first < GL_COLOR_ATTACHMENT0 + manager_->max_draw_buffers_) {
+      const Attachment* attachment = it->second.get();
+      if (!attachment->cleared())
+        return true;
+    }
+  }
+  return false;
+}
+
+void Framebuffer::ChangeDrawBuffersHelper(bool recover) const {
+  scoped_ptr<GLenum[]> buffers(new GLenum[manager_->max_draw_buffers_]);
+  for (uint32 i = 0; i < manager_->max_draw_buffers_; ++i)
+    buffers[i] = GL_NONE;
+  for (AttachmentMap::const_iterator it = attachments_.begin();
+       it != attachments_.end(); ++it) {
+    if (it->first >= GL_COLOR_ATTACHMENT0 &&
+        it->first < GL_COLOR_ATTACHMENT0 + manager_->max_draw_buffers_) {
+      buffers[it->first - GL_COLOR_ATTACHMENT0] = it->first;
+    }
+  }
+  bool different = false;
+  for (uint32 i = 0; i < manager_->max_draw_buffers_; ++i) {
+    if (buffers[i] != draw_buffers_[i]) {
+      different = true;
+      break;
+    }
+  }
+  if (different) {
+    if (recover)
+      glDrawBuffersARB(manager_->max_draw_buffers_, draw_buffers_.get());
+    else
+      glDrawBuffersARB(manager_->max_draw_buffers_, buffers.get());
+  }
+}
+
+void Framebuffer::PrepareDrawBuffersForClear() const {
+  bool recover = false;
+  ChangeDrawBuffersHelper(recover);
+}
+
+void Framebuffer::RestoreDrawBuffersAfterClear() const {
+  bool recover = true;
+  ChangeDrawBuffersHelper(recover);
+}
+
 void Framebuffer::MarkAttachmentAsCleared(
       RenderbufferManager* renderbuffer_manager,
       TextureManager* texture_manager,
@@ -516,6 +565,8 @@ void Framebuffer::SetDrawBuffers(GLsizei n, const GLenum* bufs) {
     draw_buffers_[i] = bufs[i];
 }
 
+
+
 bool Framebuffer::HasAlphaMRT() const {
   for (uint32 i = 0; i < manager_->max_draw_buffers_; ++i) {
     if (draw_buffers_[i] != GL_NONE) {