Bug 1031635 - Add --debugger-args option to reftest mach commands. r=gps

This allows useful argument combinations such as:
  --debugger valgrind
  --debugger-args "--tool=memcheck --leak-check=no --trace-children=yes --num-callers=50"

layout/tools/reftest/mach_commands.py

@@ -214,7 +214,7 @@ class ReftestRunner(MozbuildObject):
         return reftest.run_remote_reftests(parser, options, args)
 
     def run_desktop_test(self, test_file=None, filter=None, suite=None,
-            debugger=None, parallel=False, shuffle=False,
+            debugger=None, debugger_args=None, parallel=False, shuffle=False,
             e10s=False, extraPrefs=None, this_chunk=None, total_chunks=None):
         """Runs a reftest.
 
@@ -231,6 +231,8 @@ class ReftestRunner(MozbuildObject):
         debugger is the program name (in $PATH) or the full path of the
         debugger to run.
 
+        debugger_args are the arguments passed to the debugger.
+
         parallel indicates whether tests should be run in parallel or not.
 
         shuffle indicates whether to run tests in random order.
@@ -255,6 +257,14 @@ class ReftestRunner(MozbuildObject):
         if debugger:
             extra_args.append('--debugger=%s' % debugger)
             pass_thru = True
+            if debugger_args:
+                # Use _make_shell_string (which quotes) so that we
+                # handle multiple args being passed to the debugger.
+                extra_args.extend(['--debugger-args', self._make_shell_string(debugger_args)])
+        else:
+            if debugger_args:
+                print("--debugger-args passed, but no debugger specified.")
+                return 1
 
         if parallel:
             extra_args.append('--run-tests-in-parallel')
@@ -292,6 +302,10 @@ def ReftestCommand(func):
         help=DEBUGGER_HELP)
     func = debugger(func)
 
+    debugger_args = CommandArgument('--debugger-args', metavar='DEBUGGER_ARGS',
+        help='Arguments to pass to the debugger.')
+    func = debugger_args(func)
+
     flter = CommandArgument('--filter', metavar='REGEX',
         help='A JS regular expression to match test URLs against, to select '
              'a subset of tests to run.')