improve perf_basic_prof filename reporting

The buffer used for appending filenames to the string printed to the
perf_basic_prof log was unnecessarily too small. Bump it up to be at least
kUtf8BufferSize.

Truncation of filenames makes it really hard to work with profiles gathered on
Node.js. Because of the way Node.js works, you can have node module dependencies
in deeply nested directories. The last thing you want when investigating a
performance problem is to have script names be truncated.

This patch is a stop-gap. Ideally, I want no truncation of the filename at all
and use a dynamically growing buffer. That would be a larger change, and I
wanted to have a quick fix available that can be back-ported to Node.js LTS
release.

R=yangguo@chromium.org,yurys@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1388543002

Cr-Commit-Position: refs/heads/master@{#31092}

src/log.cc

@@ -145,7 +145,7 @@ class CodeEventLogger::NameBuffer {
 
  private:
   static const int kUtf8BufferSize = 512;
-  static const int kUtf16BufferSize = 128;
+  static const int kUtf16BufferSize = kUtf8BufferSize;
 
   int utf8_pos_;
   char utf8_buffer_[kUtf8BufferSize];