Default to SwiftShader as software GL implementation on MacOS with EGL

In order to enable SwiftShader as the default software GL implementation
on MacOS with EGL, a previous change had to be reverted where native
buffer formats were disabled for software GL. These formats are required
on MacOS with EGL, for IOSurface support, and SwiftShader supports the
same formats as Angle anyways, so only OSMesa needs this restriction.

Bug: chromium:726075
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: I20d9bb4083807ad9ae6d5f7092b912ea18ecb679
Reviewed-on: https://chromium-review.googlesource.com/1024500Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: Alexis Hétu <sugoi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553511}

gpu/ipc/host/gpu_memory_buffer_support.cc

@@ -17,10 +17,9 @@
 namespace gpu {
 
 bool AreNativeGpuMemoryBuffersEnabled() {
-  // Disable native buffers when using software GL.
+  // Disable native buffers when using OSMesa.
   if (base::CommandLine::ForCurrentProcess()->GetSwitchValueASCII(
-          switches::kUseGL) ==
-      gl::GetGLImplementationName(gl::GetSoftwareGLImplementation())) {
+          switches::kUseGL) == gl::kGLImplementationOSMesaName) {
     return false;
   }
 
@@ -70,11 +69,10 @@ GpuMemoryBufferConfigurationSet GetNativeGpuMemoryBufferConfigurations(
     }
   }
 
-  // Disable native buffers only when using software GL.
+  // Disable native buffers only when using OSMesa.
   bool force_native_gpu_read_write_formats =
       base::CommandLine::ForCurrentProcess()->GetSwitchValueASCII(
-          switches::kUseGL) !=
-      gl::GetGLImplementationName(gl::GetSoftwareGLImplementation());
+          switches::kUseGL) != gl::kGLImplementationOSMesaName;
   if (force_native_gpu_read_write_formats) {
     const gfx::BufferFormat kGPUReadWriteFormats[] = {
         gfx::BufferFormat::BGR_565,   gfx::BufferFormat::RGBA_8888,

ui/gl/gl_implementation.cc

@@ -122,7 +122,9 @@ GLImplementation GetNamedGLImplementation(const std::string& name) {
 }
 
 GLImplementation GetSoftwareGLImplementation() {
-#if (defined(OS_WIN) || (defined(OS_LINUX) && !defined(OS_CHROMEOS) && !defined(USE_OZONE)))
+#if (defined(OS_WIN) ||                                                     \
+     (defined(OS_LINUX) && !defined(OS_CHROMEOS) && !defined(USE_OZONE)) || \
+     (defined(OS_MACOSX) && defined(USE_EGL)))
   return kGLImplementationSwiftShaderGL;
 #else
   return kGLImplementationOSMesaGL;