profiler: Set Xhprof::$enabled after enable attempt, not before

Previously, if you tried to configure $wgProfiler to use Tideways
or XHProf, but did not actually have it (correctly) installed
as a PHP extension, then the exception shown to the developer
was "Exception: Profiling is already enabled.", whereas they should
be shown "Exception: Neither xhprof nor tideways are installed".

Here is how that happened:
* LocalSettings has $wgProfiler enabled.
* MediaWiki initialises Profiler::singleton() at some point.
  This calls Profiler -> ProfilerXhprof -> Xhprof::enable.
* Xhprof::enable() sets $enabled=true and then tries to do the
  actual enabling via callAny().
* Xhprof::callAny() finds that the user has no driver installed,
  so it throws the (correct) Exception.
* The uncaught exception is handled by MWExceptionHandler and
  results in an error page being prepared. At some point, we
  hit a service constructor that will need a Profiler, which given
  it threw earlier, does not yet have a singleton to re-use, so
  it tries again.
  This calls Profiler -> ProfilerXhprof -> Xhprof::enable.
* Xhprof::enable (hello again), now throws because its
  $enabled member was already set to true!

To fix this, simply move the assignment two lines down.

Change-Id: I89e31fe58a1c77ee2542628bec4d7bc97621ef4f

includes/libs/Xhprof.php

@@ -57,7 +57,6 @@ class Xhprof {
 			$args[] = $options;
 		}
 
-		self::$enabled = true;
 		self::callAny(
 			[
 				'xhprof_enable',
@@ -66,6 +65,7 @@ class Xhprof {
 			],
 			$args
 		);
+		self::$enabled = true;
 	}
 
 	/**