Move ReadFileToString to the base namespace.

base/process/internal_linux.cc

Index: base/process/internal_linux.cc
diff --git a/base/process/internal_linux.cc b/base/process/internal_linux.cc
index ee1107c1792aa44cf4e3a4bb3086562d98ea4820..b1d1b6f174edb206e57bd3705ae071bad9da5af4 100644
--- a/base/process/internal_linux.cc
+++ b/base/process/internal_linux.cc
@@ -54,7 +54,7 @@ bool ReadProcFile(const FilePath& file, std::string* buffer) {
   // Synchronously reading files in /proc is safe.
   ThreadRestrictions::ScopedAllowIO allow_io;
 
-  if (!file_util::ReadFileToString(file, buffer)) {
+  if (!ReadFileToString(file, buffer)) {
     DLOG(WARNING) << "Failed to read " << file.MaybeAsASCII();
     return false;
   }