Add a comment to clarify why c/b/profiles/DEPS is needed

Bug: 1168512
Change-Id: I312ad1f704621be356202b83273317f41d386986
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2639807
Auto-Submit: Sébastien Marchand <sebmarchand@chromium.org>
Commit-Queue: David Roger <droger@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845150}

chrome/browser/profiles/DEPS

 specific_include_rules = {
   "chrome_browser_main_extra_parts_profiles.cc": [
     # This is required to create the SiteDataCacheFacadeFactory when a profile
-    # is created.
+    # is created. This file isn't part of the PerformanceManager public API and
+    # its include should be explicitly allowed.
     "+chrome/browser/performance_manager/persistence/site_data/site_data_cache_facade_factory.h"
   ],
 }