Layout Tests: Fix Apache config detection for Mac 10.13 with Retina.

The _version field of Port instances is "retina" on Mac machines with
HiDPI displays. The MacPort implementation of path_to_apache_config_file
currently uses that field to switch between Apache configurations that
use PHP 7 (for 10.13) and PHP 5 (for lower versions).

The current implementation worked because Retina detection was broken on
Mac 10.12 and above. However, https://crrev.com/c/957306 fixed Retina
detection, so run-blink-httpd is currently broken on Mac 10.13 with
Retina.

This CL gets the OS version information from host.platform.os_version,
which is not overridden with "retina" on HiDPI displays.

Change-Id: I377559466c8c4d0a3e99edcc7cf1ccce49cdc123
Reviewed-on: https://chromium-review.googlesource.com/1010035Reviewed-by: Robert Ma <robertma@chromium.org>
Commit-Queue: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550337}

third_party/WebKit/Tools/Scripts/webkitpy/layout_tests/port/mac.py

@@ -94,7 +94,7 @@ class MacPort(base.Port):
 
     def path_to_apache_config_file(self):
         config_file_basename = 'apache2-httpd-' + self._apache_version()
-        if self._version == 'mac10.13':
+        if self.host.platform.os_version == 'mac10.13':
             config_file_basename += '-php7'
         return self._filesystem.join(self.apache_config_directory(), config_file_basename + '.conf')