Fix deprecation of MessageLoopProxy check

Update MessageLoopProxy deprecation regexp to match whole word only so
that calling BrowserThread::GetMessageLoopProxyForThread() or similarly
named functions do not raise the deprecation.

Followup to https://codereview.chromium.org/1097763002 to address
comments post-commit.

BUG=465354

Review URL: https://codereview.chromium.org/1099153002

Cr-Commit-Position: refs/heads/master@{#326245}
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index ba715e1..71360cb 100644
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -251,7 +251,7 @@
       ),
     ),
     (
-      'MessageLoopProxy',
+      '\<MessageLoopProxy\>',
       (
         'MessageLoopProxy is deprecated. ',
         'Please use SingleThreadTaskRunner or ThreadTaskRunnerHandle instead.'