Fix warning in string_utils.cpp in Win32

BUG=

Change-Id: Ibb25d46c2ee5f361f6a948dc3efea38422a4573f
Reviewed-on: https://chromium-review.googlesource.com/272559
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>
diff --git a/src/common/string_utils.cpp b/src/common/string_utils.cpp
index 171c501..68a96e9 100644
--- a/src/common/string_utils.cpp
+++ b/src/common/string_utils.cpp
@@ -73,7 +73,7 @@
     std::string str;
 
     inFile.seekg(0, std::ios::end);
-    str.reserve(inFile.tellg());
+    str.reserve(static_cast<std::string::size_type>(inFile.tellg()));
     inFile.seekg(0, std::ios::beg);
 
     str.assign(std::istreambuf_iterator<char>(inFile), std::istreambuf_iterator<char>());