Split PostMessageHandler from PostMessageServiceConnection

Stop inheriting from it and instead use the two classes separately, with
PostMessageServiceConnection abstracted out of PostMessageHandler behind
the PostMessageBackend interface added in change 1299016.

This means we now store a PostMessageServiceConnection in SessionParams
and call to it instead of PostMessageHandler for behaviour relating to
the service connection.

The result of this change is that PostMessageHandler can be used without
a service connection, allowing us to use it directly from CustomTabActivity
in a future CL.

Bug: 898840
Change-Id: Ifd06ee0a803ecd82c350d7b5466bf0a0ee35440b
Reviewed-on: https://chromium-review.googlesource.com/c/1299143
Commit-Queue: David Barker <[email protected]>
Reviewed-by: Michael van Ouwerkerk <[email protected]>
Reviewed-by: Peter Conn <[email protected]>
Cr-Commit-Position: refs/heads/master@{#603516}
diff --git a/DEPS b/DEPS
index 7ba5d28..418e427 100644
--- a/DEPS
+++ b/DEPS
@@ -670,7 +670,7 @@
   },
 
   'src/third_party/custom_tabs_client/src': {
-      'url': Var('chromium_git') + '/custom-tabs-client.git' + '@' + '873e69c103480c501fafdacf06e90febadeeff9c',
+      'url': Var('chromium_git') + '/custom-tabs-client.git' + '@' + 'c813ed8fcc4ece1838dba8dddf6d8ca39d6c6785',
       'condition': 'checkout_android',
   },