[dawn] Add missing 'const' on SystemEventAndReadyStateIterator operators

Change-Id: I87140420a9a2acbd31ddebf7c8bb1038ac5525c8
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/174283
Kokoro: Kokoro <noreply+kokoro@google.com>
Auto-Submit: Ben Clayton <bclayton@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: Austin Eng <enga@chromium.org>
diff --git a/src/dawn/native/EventManager.cpp b/src/dawn/native/EventManager.cpp
index c763a9e..573f346 100644
--- a/src/dawn/native/EventManager.cpp
+++ b/src/dawn/native/EventManager.cpp
@@ -113,17 +113,17 @@
 
     explicit SystemEventAndReadyStateIterator(WrappedIter wrappedIt) : mWrappedIt(wrappedIt) {}
 
-    bool operator!=(const SystemEventAndReadyStateIterator& rhs) {
+    bool operator!=(const SystemEventAndReadyStateIterator& rhs) const {
         return rhs.mWrappedIt != mWrappedIt;
     }
-    bool operator==(const SystemEventAndReadyStateIterator& rhs) {
+    bool operator==(const SystemEventAndReadyStateIterator& rhs) const {
         return rhs.mWrappedIt == mWrappedIt;
     }
-    difference_type operator-(const SystemEventAndReadyStateIterator& rhs) {
+    difference_type operator-(const SystemEventAndReadyStateIterator& rhs) const {
         return mWrappedIt - rhs.mWrappedIt;
     }
 
-    SystemEventAndReadyStateIterator operator+(difference_type rhs) {
+    SystemEventAndReadyStateIterator operator+(difference_type rhs) const {
         return SystemEventAndReadyStateIterator{mWrappedIt + rhs};
     }