webkit_layout_test: Harden stack encoding

The stack returned from _get_stack_from_dump can be None. This hardens
against this case, and hopefully another other unforseen issues around
the stack encoding, by transforming the stack to ascii inside a try
block.

Bug: 880246
Change-Id: I498b5929dab811a2ccc5e69bb51b930c8547179c
TBR: dpranke@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/1218842
Commit-Queue: Dominic Battré <battre@chromium.org>
Reviewed-by: Dominic Battré <battre@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590258}

third_party/blink/tools/blinkpy/web_tests/port/android.py

@@ -790,10 +790,14 @@ class ChromiumAndroidDriver(driver.Driver):
             crashes = self._pull_crash_dumps_from_device()
             for crash in crashes:
                 stack = self._port._dump_reader._get_stack_from_dump(crash)  # pylint: disable=protected-access
+                try:
+                  stack_str = stack.encode('ascii', 'replace')
+                except Exception as e:
+                  stack_str = '<No Stack> (%s)' % e
                 stderr += '********* [%s] breakpad minidump %s:\n%s' % (
                     self._port.host.filesystem.basename(crash),
                     self._device.serial,
-                    stack.encode('ascii', 'replace'))
+                    stack_str)
 
         return super(ChromiumAndroidDriver, self)._get_crash_log(
             stdout, stderr, newer_than)