Update ANGLE's BUILD.gn to include definitions for GL_API.

Bug: skia:
Change-Id: I57a16af6c57166ead768ad73a8124690f1fa58e4
Reviewed-on: https://skia-review.googlesource.com/98302
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
diff --git a/third_party/angle2/BUILD.gn b/third_party/angle2/BUILD.gn
index cb780c2..114a7e8 100644
--- a/third_party/angle2/BUILD.gn
+++ b/third_party/angle2/BUILD.gn
@@ -50,6 +50,7 @@
     defines += [
       "ANGLE_USE_X11",
       "GL_APICALL=__attribute__((visibility(\"default\")))",
+      "GL_API=__attribute__((visibility(\"default\")))",
       "EGLAPI=__attribute__((visibility(\"default\")))",
     ]
     cflags_cc += [ "-std=c++14" ]
@@ -58,6 +59,7 @@
       "ANGLE_ENABLE_D3D11",
       "ANGLE_ENABLE_D3D9",
       "GL_APICALL=",
+      "GL_API=",
       "EGLAPI=",
     ]