Replace base::str[n]casecmp with helper functions.

net/base/mime_util.cc

Index: net/base/mime_util.cc
diff --git a/net/base/mime_util.cc b/net/base/mime_util.cc
index 0a1774261ad45f5ba024842871cc747a2ed8e386..14b6f200c972f8ce521a24298f34578cf177ea9b 100644
--- a/net/base/mime_util.cc
+++ b/net/base/mime_util.cc
@@ -115,8 +115,11 @@ const char* FindMimeType(const MimeInfo* mappings,
     const char* extensions = mappings[i].extensions;
     for (;;) {
       size_t end_pos = strcspn(extensions, ",");
+      // The length check is required to prevent the StringPiece below from
+      // including uninitialized memory if ext is longer than extensions.
       if (end_pos == ext.size() &&
-          base::strncasecmp(extensions, ext.data(), ext.size()) == 0)
+          base::EqualsCaseInsensitiveASCII(
+              base::StringPiece(extensions, ext.size()), ext))
         return mappings[i].mime_type;
       extensions += end_pos;
       if (!*extensions)
@@ -267,13 +270,10 @@ bool MimeUtil::MatchesMimeType(const std::string& mime_type_pattern,
 
   const std::string::size_type star = base_pattern.find('*');
   if (star == std::string::npos) {
-    if (base_pattern.size() == base_type.size() &&
-        base::strncasecmp(base_pattern.data(), base_type.data(),
-                          base_pattern.size()) == 0) {
+    if (base::EqualsCaseInsensitiveASCII(base_pattern, base_type))
       return MatchesMimeTypeParameters(mime_type_pattern, mime_type);
-    } else {
+    else
       return false;
-    }
   }
 
   // Test length to prevent overlap between |left| and |right|.