Re-land^2 WebVR compositor bypass via BrowserMain context + mailbox

chrome/browser/android/vr_shell/vr_shell_renderer.cc

Index: chrome/browser/android/vr_shell/vr_shell_renderer.cc
diff --git a/chrome/browser/android/vr_shell/vr_shell_renderer.cc b/chrome/browser/android/vr_shell/vr_shell_renderer.cc
index 55ce951060754346029617c78f5237602d2067bf..0ce95a9f712eda808e3f8c369c9094bd352a833b 100644
--- a/chrome/browser/android/vr_shell/vr_shell_renderer.cc
+++ b/chrome/browser/android/vr_shell/vr_shell_renderer.cc
@@ -406,11 +406,9 @@ void WebVrRenderer::Draw(int texture_handle) {
                         VOID_OFFSET(kTextureCoordinateDataOffset));
   glEnableVertexAttribArray(tex_coord_handle_);
 
-  // Bind texture. Ideally this should be a 1:1 pixel copy. (Or even more
-  // ideally, a zero copy reuse of the texture.) For now, we're using an
-  // undersized render target for WebVR, so GL_LINEAR makes it look slightly
-  // less chunky. TODO(klausw): change this to GL_NEAREST once we're doing
-  // a 1:1 copy since that should be more efficient.
+  // Bind texture. This is a 1:1 pixel copy since the source surface
+  // and renderbuffer destination size are resized to match, so use
+  // GL_NEAREST.
   glActiveTexture(GL_TEXTURE0);
   glBindTexture(GL_TEXTURE_EXTERNAL_OES, texture_handle);
   glTexParameteri(GL_TEXTURE_EXTERNAL_OES, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);