Revert "[logging] Remove unused code"

This reverts commit c32f6614.

Reason for revert:
https://build.chromium.org/p/client.v8/builders/V8%20Linux64%20TSAN/builds/20563

Original change's description:
> [logging] Remove unused code
> 
> This removes unused mutex, headers from log.(h|cc).
> 
> Change-Id: Ie4dc69a7efa9494b21ff6e2b19828bd740e3a47c
> Reviewed-on: https://chromium-review.googlesource.com/1041967
> Reviewed-by: Camillo Bruni <cbruni@chromium.org>
> Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#52946}

TBR=jarin@chromium.org,cbruni@chromium.org

Change-Id: I48b63c7fc6a10e118b735f708db37d353ba6c0f7
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/1041969Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52950}

src/log-utils.cc

@@ -82,7 +82,8 @@ FILE* Log::Close() {
   return result;
 }
 
-Log::MessageBuilder::MessageBuilder(Log* log) : log_(log) {
+Log::MessageBuilder::MessageBuilder(Log* log)
+    : log_(log), lock_guard_(&log_->mutex_) {
   DCHECK_NOT_NULL(log_->format_buffer_);
 }
 

src/log-utils.h

@@ -7,6 +7,11 @@
 
 #include <stdio.h>
 
+#include <cstdarg>
+
+#include "src/allocation.h"
+#include "src/base/compiler-specific.h"
+#include "src/base/platform/mutex.h"
 #include "src/flags.h"
 #include "src/ostreams.h"
 
@@ -90,6 +95,7 @@ class Log {
 
    private:
     Log* log_;
+    base::LockGuard<base::Mutex> lock_guard_;
   };
 
  private:
@@ -111,6 +117,10 @@ class Log {
   FILE* output_handle_;
   OFStream os_;
 
+  // mutex_ is a Mutex used for enforcing exclusive
+  // access to the formatting buffer and the log file or log memory buffer.
+  base::Mutex mutex_;
+
   // Buffer used for formatting log messages. This is a singleton buffer and
   // mutex_ should be acquired before using it.
   char* format_buffer_;