Bug 1602717 - Re-enable stackwalking on local macOS builds. r=erahm

Now that `fix_stacks.py` is being used instead of `fix_macosx_stack.py`,
stack-fixing time has dropped from about 14 minutes to about 30 seconds on my
new MacBook Pro.

Also, print a warning about stacks not being fixed if `MOZ_DISABLE_STACK_FIX`
is set. This warning shows up at the start of the test run.

Also, print a warning about stack fixing slowness, because 30 seconds is long
enough to possibly be surprising. This warning shows up just before the first
stack frame is fixed, like this:
```
Assertion failure: false (BEEP BOOP), at /home/njn/moz/au3/dom/base/nsGlobalWindowOuter.cpp:1342
Initializing stack-fixing for the first stack frame, this may take a while...
#1: nsGlobalWindowOuter::nsGlobalWindowOuter(unsigned long) (/home/njn/moz/au3/dom/base/nsGlobalWindowOuter.cpp:1342)
#2: ...
```

Differential Revision: https://phabricator.services.mozilla.com/D65676

UltraBlame original commit: 9d6c366e7cde369c65ddb85b258dbacbf396e65e

testing/mozbase/mozrunner/mozrunner/utils.py

@@ -237,9 +237,7 @@ def get_stack_fixer_function(utilityPath, symbolsPath):
         return None
 
     if os.getenv('MOZ_DISABLE_STACK_FIX', 0):
-        return None
-
-    if mozinfo.isMac and not mozinfo.automation:
+        print("WARNING: No stack-fixing will occur because MOZ_DISABLE_STACK_FIX is set")
         return None
 
     def import_stack_fixer_module(module_name):
@@ -265,7 +263,7 @@ def stack_fixer_function(line):
         stack_fixer_module = import_stack_fixer_module('fix_stacks')
 
         def stack_fixer_function(line):
-            return stack_fixer_module.fixSymbols(line)
+            return stack_fixer_module.fixSymbols(line, slowWarning=True)
 
     else:
         return None

tools/rb/fix_stacks.py

@@ -7,7 +7,7 @@
 
 
 
-from __future__ import absolute_import
+from __future__ import absolute_import, print_function
 from subprocess import Popen, PIPE
 import os
 import platform
@@ -21,7 +21,7 @@
 fix_stacks = None
 
 
-def fixSymbols(line, jsonMode=False):
+def fixSymbols(line, jsonMode=False, slowWarning=False):
     global fix_stacks
 
     result = line_re.search(line)
@@ -52,6 +52,9 @@ def fixSymbols(line, jsonMode=False):
 
         fix_stacks = Popen(args, stdin=PIPE, stdout=PIPE, stderr=None)
 
+        if slowWarning:
+            print("Initializing stack-fixing for the first stack frame, this may take a while...")
+