[fuchsia] Save system log from log_listener to web test output.

This saves log_listener output from blink web tests running on Fuchsia
to the system.log file in the test output isolate, for better debugging
of errors inside the Fuchsia VM.

Bug: 1055553
Change-Id: I7c3ce011dfd11d381532511bbe53e505e785284d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2086052Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Stephen Roe <steveroe@google.com>
Cr-Commit-Position: refs/heads/master@{#746878}

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

@@ -125,7 +125,8 @@ class SubprocessOutputLogger(object):
         self._process.kill()
 
 class _TargetHost(object):
-    def __init__(self, build_path, ports_to_forward, target_device):
+    def __init__(self, build_path, ports_to_forward, target_device,
+                 results_directory):
         try:
             self._target = None
             target_args = {
@@ -146,12 +147,12 @@ class _TargetHost(object):
                 })
                 self._target = aemu_target.AemuTarget(**target_args)
             self._target.Start()
-            self._setup_target(build_path, ports_to_forward)
+            self._setup_target(build_path, ports_to_forward, results_directory)
         except:
             self.cleanup()
             raise
 
-    def _setup_target(self, build_path, ports_to_forward):
+    def _setup_target(self, build_path, ports_to_forward, results_directory):
         # Tell SSH to forward all server ports from the Fuchsia device to
         # the host.
         forwarding_flags = [
@@ -165,6 +166,12 @@ class _TargetHost(object):
                                                    ssh_args=forwarding_flags,
                                                    stderr=subprocess.PIPE)
 
+        listener_log_path = os.path.join(results_directory, 'system.log')
+        listener_log = open(listener_log_path,'w')
+        self._listener = self._target.RunCommandPiped(['log_listener'],
+                                                    stderr=listener_log,
+                                                    stdout=listener_log)
+
         package_path = os.path.join(build_path, CONTENT_SHELL_PACKAGE_PATH)
         self._target.InstallPackage([package_path])
 
@@ -228,7 +235,8 @@ class FuchsiaPort(base.Port):
         super(FuchsiaPort, self).setup_test_run()
         try:
             self._target_host = _TargetHost(
-                self._build_path(), self.SERVER_PORTS, self._target_device)
+                self._build_path(), self.SERVER_PORTS, self._target_device,
+                self.results_directory())
 
             if self.get_option('zircon_logging'):
                 self._zircon_logger = SubprocessOutputLogger(