Heap profiling: add logging of heap memory stats (capacity, used) under 'log-gc' flag.

tools/process-heap-prof.py

Index: tools/process-heap-prof.py
diff --git a/tools/process-heap-prof.py b/tools/process-heap-prof.py
index 79c49f83d8ca300d2aa3f2f16ac5ff7cc0b05e2b..b8ab2d39863e345d7572f6a6d541a25d32401eb9 100755
--- a/tools/process-heap-prof.py
+++ b/tools/process-heap-prof.py
@@ -39,6 +39,7 @@
 import csv, sys, time
 
 def process_logfile(filename):
+  first_call_time = None
   sample_time = 0.0
   sampling = False
   try:
@@ -53,7 +54,10 @@ def process_logfile(filename):
 
       for row in logreader:
         if row[0] == 'heap-sample-begin' and row[1] == 'Heap':
-          sample_time = float(row[3]) + float(row[4])/1000000.0
+          sample_time = float(row[3])/1000.0
+          if first_call_time == None:
+            first_call_time = sample_time
+          sample_time -= first_call_time
           print('BEGIN_SAMPLE %.2f' % sample_time)
           sampling = True
         elif row[0] == 'heap-sample-end' and row[1] == 'Heap':