[Chromecast] Disable browser-side navigation by default on cast_shell.

This is a new feature that is enabled by the network service
(http://crbug.com/689549). A recent change landed on master, causing a
DCHECK at runtime, and blocking the roll: http://crrev.com/c/581488.

Disable this feature by default while we work with clamy@ to add
support.

BUG=768526

Bug: b/66275254 (Chromium merge for Cast)
Test: Run cast_shell on desktop, Cast YT.
Change-Id: I35323e01215b2923d0f24130dbb423c0cd55018d
Reviewed-on: https://chromium-review.googlesource.com/682955Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Commit-Queue: Stephen Lanham <slan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504188}

chromecast/browser/cast_browser_main_parts.cc

@@ -282,6 +282,13 @@ void AddDefaultCommandLineSwitches(base::CommandLine* command_line) {
       VLOG(2) << "Skip setting default switch '" << name << "', already set";
     }
   }
+
+  // If browser-side navigation is not explicitly enabled or disabled, disable
+  // it.
+  if (!command_line->HasSwitch(switches::kDisableBrowserSideNavigation) &&
+      !command_line->HasSwitch(switches::kEnableBrowserSideNavigation)) {
+    command_line->AppendSwitch(switches::kDisableBrowserSideNavigation);
+  }
 }
 
 }  // namespace