Move SkGLContext and some GrGLInterface implementations to skgputest module

BUG=skia:
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Committed: https://skia.googlesource.com/skia/+/fe3456cb006110d045b26ff3f8681b893a757b58

Review URL: https://codereview.chromium.org/1815823002
diff --git a/tools/flags/SkCommonFlagsConfig.cpp b/tools/flags/SkCommonFlagsConfig.cpp
index 9e0fd60..1ee3130 100644
--- a/tools/flags/SkCommonFlagsConfig.cpp
+++ b/tools/flags/SkCommonFlagsConfig.cpp
@@ -9,6 +9,10 @@
 
 #include <stdlib.h>
 
+#if SK_SUPPORT_GPU
+using sk_gpu_test::GrContextFactory;
+#endif
+
 static const char defaultConfigs[] =
     "565 8888 gpu nonrendering"
 #if SK_ANGLE