Fix include what you use issue in atomic_string_hash.h.

I encountered trybot failures in https://crrev.com/c/1907147 -- I was able to
fix them by including both atomic_string_hash.h and string_hash.h, but
atomic_string_hash.h should IWYU.

Change-Id: I2501300bdb85bd765b8af6e9605dea460c26adc2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1924981
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Reviewed-by: Yuta Kitamura <yutak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#716950}

third_party/blink/renderer/platform/wtf/text/atomic_string_hash.h

@@ -31,6 +31,7 @@
 
 #include "third_party/blink/renderer/platform/wtf/hash_traits.h"
 #include "third_party/blink/renderer/platform/wtf/text/atomic_string.h"
+#include "third_party/blink/renderer/platform/wtf/text/string_hash.h"
 
 namespace WTF {