Fix iterator comparison in Context.cpp.

In deleteTransformFeedback, we were using a const iterator, in a
non-const method. Fix the comparison by using auto, which delegates
more work to the compiler's static type checking.

BUG=angle:773

Change-Id: Ibc362fe49629331811a5302df1b68269dfa128a6
Reviewed-on: https://chromium-review.googlesource.com/238444
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
diff --git a/src/libANGLE/Context.cpp b/src/libANGLE/Context.cpp
index fa997dc..a2a8261 100644
--- a/src/libANGLE/Context.cpp
+++ b/src/libANGLE/Context.cpp
@@ -363,7 +363,7 @@
 
 void Context::deleteTransformFeedback(GLuint transformFeedback)
 {
-    TransformFeedbackMap::const_iterator iter = mTransformFeedbackMap.find(transformFeedback);
+    auto iter = mTransformFeedbackMap.find(transformFeedback);
     if (iter != mTransformFeedbackMap.end())
     {
         detachTransformFeedback(transformFeedback);