IWYU: Include <cmath> for std::isnan()

This fixes the libstdc++ build after commit 4b6b8cf6 ("[cleanup] Start
cleaning up utils.h"), as string-util.cc was including <cmath> indirectly
via src/conversions.h.

Bug: chromium:819294
Change-Id: I5be7eac355d0c316a432eab4c50ae66d885b03d7
Reviewed-on: https://chromium-review.googlesource.com/c/1491591Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59905}

src/inspector/string-util.cc

@@ -4,6 +4,8 @@
 
 #include "src/inspector/string-util.h"
 
+#include <cmath>
+
 #include "src/base/platform/platform.h"
 #include "src/conversions.h"
 #include "src/inspector/protocol/Protocol.h"