Do not use std::set::at in Config.cpp.

This is C++11 STL.

BUG=459399

Change-Id: I2e994e6b47535bcb38c193db2aaecc304e388772
Reviewed-on: https://chromium-review.googlesource.com/250581
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>
diff --git a/src/libANGLE/Config.cpp b/src/libANGLE/Config.cpp
index c9bd12c..e89a1c2 100644
--- a/src/libANGLE/Config.cpp
+++ b/src/libANGLE/Config.cpp
@@ -75,7 +75,8 @@
 
 const Config &ConfigSet::get(EGLint id) const
 {
-    return mConfigs.at(id);
+    ASSERT(mConfigs.find(id) != mConfigs.end());
+    return mConfigs.find(id)->second;
 }
 
 void ConfigSet::clear()