Initial version of external_oes texture support and unit test

src/gpu/glsl/GrGLSLCaps.cpp

Index: src/gpu/glsl/GrGLSLCaps.cpp
diff --git a/src/gpu/glsl/GrGLSLCaps.cpp b/src/gpu/glsl/GrGLSLCaps.cpp
index aed98880f5af73d1eeaeeb7293b707f2ed0b0a6f..191fb567e6d87b52247b5cdc4bf4863ab34b3f84 100755
--- a/src/gpu/glsl/GrGLSLCaps.cpp
+++ b/src/gpu/glsl/GrGLSLCaps.cpp
@@ -28,6 +28,7 @@ GrGLSLCaps::GrGLSLCaps(const GrContextOptions& options) {
     fShaderDerivativeExtensionString = nullptr;
     fFragCoordConventionsExtensionString = nullptr;
     fSecondaryOutputExtensionString = nullptr;
+    fExternalTextureExtensionString = nullptr;
     fFBFetchColorName = nullptr;
     fFBFetchExtensionString = nullptr;
     fAdvBlendEqInteraction = kNotSupported_AdvBlendEqInteraction;