Change the NaCl loader and broker processes to use the ServiceManager.

This is the first child process outside of content to be connected to
the ServiceManager so this CL adds a way for content embedders to
provide additional service manifests via ContentBrowserClient. This
would introduce an unnecessary string copy when obtaining service
manifest overlays, so this changes that API to take a StringPiece.

BUG=666605
[email protected]

Review-Url: https://codereview.chromium.org/2501913002
Cr-Commit-Position: refs/heads/master@{#440299}
diff --git a/content/BUILD.gn b/content/BUILD.gn
index 85f4a51..33c2d22 100644
--- a/content/BUILD.gn
+++ b/content/BUILD.gn
@@ -92,6 +92,7 @@
       "public/common/content_switches.cc",
       "public/common/content_switches.h",
       "public/common/mojo_channel_switches.cc",
+      "public/common/mojo_channel_switches.h",
     ]
     set_sources_assignment_filter(sources_assignment_filter)