Prevent implicit framebuffer clear from clobbering alpha in emulated RGB

third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp

Index: third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp
diff --git a/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp b/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp
index a909bea5d88abdf02dd70dd231282cd19247ecc8..5ac50c4acb226cae15258341c1da6907b007fa0c 100644
--- a/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp
+++ b/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp
@@ -987,6 +987,7 @@ void DrawingBuffer::resolveMultisampleFramebufferInternal() {
             .disable_multisampling_color_mask_usage) {
       m_gl->ClearColor(0, 0, 0, 1);
       m_gl->ColorMask(false, false, false, true);
+      m_gl->Clear(GL_COLOR_BUFFER_BIT);
     }
   }