Add 32-bit hash helper to ImmutableString

Since the length of the hash is known prior to compilation, it can be
compared with script-generated hashes.

BUG=angleproject:2267
TEST=angle_unittests

Change-Id: Ia0a78dfd450c4ea2d526da7f3495b9750dcbd1af
Reviewed-on: https://chromium-review.googlesource.com/931884
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
diff --git a/src/compiler/translator/ImmutableString.h b/src/compiler/translator/ImmutableString.h
index e077377..301e91c 100644
--- a/src/compiler/translator/ImmutableString.h
+++ b/src/compiler/translator/ImmutableString.h
@@ -128,6 +128,8 @@
         }
     };
 
+    uint32_t hash32() const;
+
   private:
     const char *mData;
     size_t mLength;