Bug 1239552 - Disable additional welcome URL in automation; r=automatedtester

The preference startup.homepage_welcome_url.additional causes problems
with official branded builds and breaks certain Marionette tests, such as
testing/marionette/harness/marionette/tests/unit/test_window_handles.py
and testing/marionette/harness/marionette/tests/unit/test_selected.py.

Many Marionette tests make assumptions about the number of open windows,
and this aligns the test environment of the officially branded builds
to be more similar to that of Nightly builds.

There is another issue in Marionette, described in bug 1239552, that we
register the listener IPC message handlers twice.  This patch does not
directly address that problem.

MozReview-Commit-ID: KEdJEcOxyGy

--HG--
extra : rebase_source : 48c61804618c76dc0b69abf94421b7e45f4ddfb0

testing/marionette/client/marionette_driver/geckoinstance.py

@@ -37,7 +37,8 @@ class GeckoInstance(object):
         "extensions.autoDisableScopes": 10,
         "focusmanager.testmode": True,
         "marionette.defaultPrefs.enabled": True,
-        "startup.homepage_welcome_url": "about:blank",
+        "startup.homepage_welcome_url": "",
+        "startup.homepage_welcome_url.additional": "",
         "toolkit.telemetry.enabled": False,
         # Until Bug 1238095 is fixed, we have to enable CPOWs in order
         # for Marionette tests to work properly.
@@ -298,20 +299,22 @@ class DesktopInstance(GeckoInstance):
         'app.update.auto': False,
         'app.update.enabled': False,
         'browser.dom.window.dump.enabled': True,
+        'browser.firstrun-content.dismissed': True,
         # Bug 1145668 - Has to be reverted to about:blank once Marionette
         # can correctly handle error pages
         'browser.newtab.url': 'about:newtab',
         'browser.newtabpage.enabled': False,
         'browser.reader.detectedFirstArticle': True,
-        'browser.safebrowsing.phishing.enabled': False,
+        'browser.safebrowsing.blockedURIs.enabled': False,
         'browser.safebrowsing.forbiddenURIs.enabled': False,
         'browser.safebrowsing.malware.enabled': False,
-        'browser.safebrowsing.blockedURIs.enabled': False,
+        'browser.safebrowsing.phishing.enabled': False,
         'browser.search.update': False,
         'browser.tabs.animate': False,
         'browser.tabs.warnOnClose': False,
         'browser.tabs.warnOnOpen': False,
         'browser.uitour.enabled': False,
+        'browser.usedOnWindows10.introURL': '',
         'extensions.getAddons.cache.enabled': False,
         'extensions.installDistroAddons': False,
         'extensions.showMismatchUI': False,