Fix more disabled MSVC warnings, base/ edition.

base/files/file_util.cc

Index: base/files/file_util.cc
diff --git a/base/files/file_util.cc b/base/files/file_util.cc
index 96a7164720d1ee19609135a792aa8fed6cd130b4..17b596946baadcd59928e4ce179c9b0aa25348b8 100644
--- a/base/files/file_util.cc
+++ b/base/files/file_util.cc
@@ -75,7 +75,7 @@ bool ContentsEqual(const FilePath& filename1, const FilePath& filename2) {
 
     if ((file1.eof() != file2.eof()) ||
         (file1.gcount() != file2.gcount()) ||
-        (memcmp(buffer1, buffer2, file1.gcount()))) {
+        (memcmp(buffer1, buffer2, static_cast<size_t>(file1.gcount())))) {
       file1.close();
       file2.close();
       return false;