deps: backport 2bcbe2f from V8 upstream

Excessive buffering of perf map files in V8 could cause profiles
to be missing symbols at times.

Original commit message:
  switch perf and ll_prof loggers to line buffering

  BUG=v8:5015
  R=jarin@chromium.org,yangguo@chromium.org

  Review-Url: https://codereview.chromium.org/2041243002
  Cr-Commit-Position: refs/heads/master@{#36788}

PR-URL: https://github.com/nodejs/node/pull/7814
Reviewed-By: bnoordhuis - Ben Noordhuis <info@bnoordhuis.nl>

deps/v8/include/v8-version.h

@@ -11,7 +11,7 @@
 #define V8_MAJOR_VERSION 4
 #define V8_MINOR_VERSION 5
 #define V8_BUILD_NUMBER 103
-#define V8_PATCH_LEVEL 41
+#define V8_PATCH_LEVEL 42
 
 // Use 1 for candidates and 0 otherwise.
 // (Boolean macro values are not supported by all preprocessors.)

deps/v8/src/log.cc

@@ -250,10 +250,6 @@ class PerfBasicLogger : public CodeEventLogger {
   static const char kFilenameFormatString[];
   static const int kFilenameBufferPadding;
 
-  // File buffer size of the low-level log. We don't use the default to
-  // minimize the associated overhead.
-  static const int kLogBufferSize = 2 * MB;
-
   FILE* perf_output_handle_;
 };
 
@@ -274,7 +270,7 @@ PerfBasicLogger::PerfBasicLogger()
   perf_output_handle_ =
       base::OS::FOpen(perf_dump_name.start(), base::OS::LogFileOpenMode);
   CHECK_NOT_NULL(perf_output_handle_);
-  setvbuf(perf_output_handle_, NULL, _IOFBF, kLogBufferSize);
+  setvbuf(perf_output_handle_, NULL, _IOLBF, 0);
 }
 
 
@@ -361,10 +357,6 @@ class LowLevelLogger : public CodeEventLogger {
   // Extension added to V8 log file name to get the low-level log name.
   static const char kLogExt[];
 
-  // File buffer size of the low-level log. We don't use the default to
-  // minimize the associated overhead.
-  static const int kLogBufferSize = 2 * MB;
-
   void LogCodeInfo();
   void LogWriteBytes(const char* bytes, int size);
 
@@ -389,7 +381,7 @@ LowLevelLogger::LowLevelLogger(const char* name)
   MemCopy(ll_name.start() + len, kLogExt, sizeof(kLogExt));
   ll_output_handle_ =
       base::OS::FOpen(ll_name.start(), base::OS::LogFileOpenMode);
-  setvbuf(ll_output_handle_, NULL, _IOFBF, kLogBufferSize);
+  setvbuf(ll_output_handle_, NULL, _IOLBF, 0);
 
   LogCodeInfo();
 }