A couple fixes to find_unmapped_histograms.py.

tools/metrics/histograms/find_unmapped_histograms.py

Index: tools/metrics/histograms/find_unmapped_histograms.py
diff --git a/tools/metrics/histograms/find_unmapped_histograms.py b/tools/metrics/histograms/find_unmapped_histograms.py
index 443d1c6fa637413f68099e60d8f146bb65db4506..31e41b5066fd2a4a4f5ac4b828af44b57dd743c1 100644
--- a/tools/metrics/histograms/find_unmapped_histograms.py
+++ b/tools/metrics/histograms/find_unmapped_histograms.py
@@ -38,8 +38,8 @@ HISTOGRAM_REGEX = re.compile(r"""
     \w*            # Match the rest of the macro name, e.g. '_ENUMERATION'
     \(             # Match the opening parenthesis for the macro
     \s*            # Match any whitespace -- especially, any newlines
-    ([^,]*)        # Capture the first parameter to the macro
-    ,              # Match the comma that delineates the first parameter
+    ([^,)]*)       # Capture the first parameter to the macro
+    [,)]           # Match the comma/paren that delineates the first parameter
     """, re.VERBOSE)
 
 
@@ -113,10 +113,6 @@ def logNonLiteralHistogram(filename, histogram):
   if '\\' in histogram:
     return
 
-  # Field trials are unique within a session, so are effectively constants.
-  if histogram.startswith('base::FieldTrial::MakeName'):
-    return
-
   # Ignore histogram names that have been pulled out into C++ constants.
   if CONSTANT_REGEX.match(histogram):
     return