Define SK_GL in Skia build.

Historically Skia's GL backend was always built and other backends were
opt-in. Skia is making GL opt-in as well to support Vulkan-only and
Metal-only builds of Skia.

Also removes definition of GR_GL_IGNORE_ES3_MSAA which no longer has any
meaning in Skia.

Change-Id: I0dbcd3fc961258e927397b6b9e132d81b13c99df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1628807Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
Cr-Commit-Position: refs/heads/master@{#663129}

skia/BUILD.gn

@@ -39,6 +39,7 @@ config("skia_config") {
   }
 
   defines = [
+    "SK_GL",
     "SK_HAS_PNG_LIBRARY",
     "SK_HAS_WEBP_LIBRARY",
     "SK_USER_CONFIG_HEADER=\"../../skia/config/SkUserConfig.h\"",
@@ -55,10 +56,7 @@ config("skia_config") {
   }
 
   if (is_component_build) {
-    defines += [
-      "SKIA_DLL",
-      "GR_GL_IGNORE_ES3_MSAA=0",
-    ]
+    defines += [ "SKIA_DLL" ]
     if (is_win) {
       defines += [ "SKCMS_API=__declspec(dllexport)" ]
     } else {