exo: Disable RGBX fullscreen overlays.

Fullscreen overlays have been enabled on minnie.
Unfortunately there are some issues in HardwareDisplayPlaneManager::
AssignOverlayPlanes with RGBX buffers.

This patch disables the overlay optimization for those buffers
while we work on a fix.

BUG=b/29430506

Review-Url: https://codereview.chromium.org/2278803003
Cr-Commit-Position: refs/heads/master@{#414281}

components/exo/display.cc

@@ -90,8 +90,8 @@ std::unique_ptr<Buffer> Display::CreateLinuxDMABufBuffer(
 
   // List of overlay formats that are known to be supported.
   // TODO(reveman): Determine this at runtime.
-  const gfx::BufferFormat kOverlayFormats[] = {gfx::BufferFormat::RGBA_8888,
-                                               gfx::BufferFormat::RGBX_8888};
+  // TODO(dcastagna): Re-add RGBX_8888 format.
+  const gfx::BufferFormat kOverlayFormats[] = {gfx::BufferFormat::RGBA_8888};
   bool is_overlay_candidate =
       std::find(std::begin(kOverlayFormats), std::end(kOverlayFormats),
                 format) != std::end(kOverlayFormats);