More SynchronousInterfacePtr conversion + some related pexe content handler cleanup.

R=smklein@chromium.org, vardhan@google.com

Review URL: https://codereview.chromium.org/1932713003 .
diff --git a/shell/BUILD.gn b/shell/BUILD.gn
index 23b7dc6..028ab49 100644
--- a/shell/BUILD.gn
+++ b/shell/BUILD.gn
@@ -673,6 +673,7 @@
     "//mojo/public/interfaces/application",
     "//mojo/services/http_server/cpp",
     "//mojo/services/http_server/interfaces",
+    "//mojo/services/http_server/interfaces:interfaces_sync",
     "//mojo/services/network/interfaces",
     "//shell/test:bindings",
   ]