Fix clearing of implicit stencil buffer.

This was lost in the recent DrawingBuffer refactoring adding
clearBuffer support, leading to performance losses on lower-end GPUs.

Bug: 834500
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: Ic81ac215183419145e7e9a67552e760cd3854764
Reviewed-on: https://chromium-review.googlesource.com/1020527Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#552234}

third_party/blink/renderer/platform/graphics/gpu/drawing_buffer.cc

@@ -208,7 +208,7 @@ void DrawingBuffer::ResetBuffersToAutoClear() {
   GLuint buffers = GL_COLOR_BUFFER_BIT;
   if (want_depth_)
     buffers |= GL_DEPTH_BUFFER_BIT;
-  if (want_stencil_)
+  if (want_stencil_ || has_implicit_stencil_buffer_)
     buffers |= GL_STENCIL_BUFFER_BIT;
   SetBuffersToAutoClear(buffers);
 }