Drop the default MSAA samples to 2x for WebXR on mobile

This makes it easier to hit the 60FPS ideal while encouraging apps to
use some antialiasing by default. Should be paired with a future CL
that bumps the default buffer resolution slightly to help compensate
for the quality loss.

Bug: 832185
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I7f089b4cde6c2d55d3965a4927d05a2a74520e2b
Reviewed-on: https://chromium-review.googlesource.com/1010774Reviewed-by: Klaus Weidner <klausw@chromium.org>
Reviewed-by: Ian Vollick <vollick@chromium.org>
Commit-Queue: Ian Vollick <vollick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550378}

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

@@ -4,6 +4,7 @@
 
 #include "third_party/blink/renderer/platform/graphics/gpu/xr_webgl_drawing_buffer.h"
 
+#include "build/build_config.h"
 #include "third_party/blink/renderer/platform/graphics/accelerated_static_bitmap_image.h"
 #include "third_party/blink/renderer/platform/graphics/gpu/drawing_buffer.h"
 #include "third_party/blink/renderer/platform/graphics/gpu/extensions_3d_util.h"
@@ -145,7 +146,14 @@ bool XRWebGLDrawingBuffer::Initialize(const IntSize& size,
       (drawing_buffer_->webgl_version() > DrawingBuffer::kWebGL1 ||
        extensions_util->SupportsExtension("GL_EXT_texture_storage")) &&
       anti_aliasing_mode_ == kScreenSpaceAntialiasing;
+
+#if defined(OS_ANDROID)
+  // On Android devices use a smaller numer of samples to provide more breathing
+  // room for fill-rate-bound applications.
+  sample_count_ = std::min(2, max_sample_count);
+#else
   sample_count_ = std::min(4, max_sample_count);
+#endif
 
   Resize(size);