Don't use SwANGLE for headless yet

There are a number of issues related to using SwANGLE with
headless Ozone, and I need more time to debug them. This CL
turns off SwANGLE for headless Ozone for now, until all
headless related issues are fixed.

Bug: chromium:1228219
Bug: chromium:1227864
Change-Id: I127ef0d5f05b48edcbea01eb7df76c48019e041c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3021045
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Alexis Hétu <sugoi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#900559}

ui/gl/init/gl_factory.cc

@@ -142,7 +142,8 @@ GLImplementationParts GetSoftwareGLForTestsImplementation() {
 #if defined(OS_WIN) || defined(OS_LINUX)
 #if defined(USE_OZONE)
   if (!features::IsUsingOzonePlatform() ||
-      (ui::OzonePlatform::GetPlatformNameForTest() != "wayland"))
+      ((ui::OzonePlatform::GetPlatformNameForTest() != "wayland") &&
+       (ui::OzonePlatform::GetPlatformNameForTest() != "headless")))
 #endif
   {
     return GetSoftwareGLImplementation();