[cros search service]: Include <cmath> for std::pow()

IWYU. Follow up to commit 2b2ea3c0 ("[cros search service] Move shared
string matching functions to //chrome"), which broke the libstdc++ build:

    ../../chrome/common/string_matching/fuzzy_tokenized_string_match.cc:199:14: error: no member named 'pow' in namespace 'std'
            std::pow(partial_match_penalty_rate, long_start - current - 1);
            ~~~~~^

Bug: 957519
Change-Id: I66f61cb4f93cfa0bfa3d1b00ba391ddd8f31a7fb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1960310
Auto-Submit: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Reviewed-by: Jia Meng <jiameng@chromium.org>
Commit-Queue: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Cr-Commit-Position: refs/heads/master@{#723499}

chrome/common/string_matching/fuzzy_tokenized_string_match.cc

@@ -5,6 +5,7 @@
 #include "chrome/common/string_matching/fuzzy_tokenized_string_match.h"
 
 #include <algorithm>
+#include <cmath>
 #include <iterator>
 
 #include "base/i18n/case_conversion.h"