Don't use exact-width atomics in blink.

Exact-width atomics are C++17 feature while chromium and blink are
still supposed to target C++14. The code builds fine when using default
bundled libcxx, but fails when using external C++ standard library that
does not support C++17.

Ref: http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#2441
Change-Id: Ia83342a7920e19d5d270e17aa49da39730084713
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2438458Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Piotr Tworek <ptworek@vewd.com>
Cr-Commit-Position: refs/heads/master@{#812596}

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

@@ -543,7 +543,7 @@ class WTF_EXPORT StringImpl {
 #endif
   mutable unsigned ref_count_{1};
   const unsigned length_;
-  mutable std::atomic_uint32_t hash_and_flags_;
+  mutable std::atomic<uint32_t> hash_and_flags_;
 
   DISALLOW_COPY_AND_ASSIGN(StringImpl);
 };