调整gperftools中Heap Profiler输出字符串缓冲区大小到1<<22，并升级gperftools.

* heap profiler工具中定义的Heap文件输出缓冲区过小（1<<20），当使用内存较多时无法正确输出Heap文件。通过Patch的方式调整。
  * 升级gperftools到最新稳定版本2.8。

vendor/patch/gperftools-fix_too_small_heap_profile_buffer_size.patch

@@ -0,0 +1,13 @@
+diff --git a/src/heap-profiler.cc b/src/heap-profiler.cc
+index 33a25ac..4d750bb 100644
+--- a/src/heap-profiler.cc
++++ b/src/heap-profiler.cc
+@@ -151,7 +151,7 @@ static void ProfilerFree(void* p) {
+ }
+ 
+ // We use buffers of this size in DoGetHeapProfile.
+-static const int kProfileBufferSize = 1 << 20;
++static const int kProfileBufferSize = 1 << 22;
+ 
+ // This is a last-ditch buffer we use in DumpProfileLocked in case we
+ // can't allocate more memory from ProfilerMalloc.  We expect this