Implement EXT_shader_texture_lod

BUG=angle:551

Change-Id: I81d7574a15861f1b24ddf6147cf71adbf20e10f3
Reviewed-on: https://chromium-review.googlesource.com/194960
Tested-by: Nicolas Capens <nicolascapens@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
diff --git a/src/libGLESv2/Context.cpp b/src/libGLESv2/Context.cpp
index a7039e9..507af51 100644
--- a/src/libGLESv2/Context.cpp
+++ b/src/libGLESv2/Context.cpp
@@ -3812,6 +3812,7 @@
 
         mExtensionStringList.push_back("GL_EXT_read_format_bgra");
         mExtensionStringList.push_back("GL_EXT_robustness");
+        mExtensionStringList.push_back("GL_EXT_shader_texture_lod");
 
         if (supportsDXT1Textures())
         {