ResourceLoadingHints turnon work (flag and bug fix)

Adds about:flags support for ResourceLoadingHints and fixes an
issue for determining the initial PreviewsState (where we need
to "allow" ResourceLoadingHints preview even if no hints loaded
for it yet (will be checked at commit time).

Bug: 871795
Change-Id: I52529a29dabf6e5be23c49bf7f597783418d053b
Reviewed-on: https://chromium-review.googlesource.com/1187574
Commit-Queue: Doug Arnett <[email protected]>
Reviewed-by: Ryan Sturm <[email protected]>
Cr-Commit-Position: refs/heads/master@{#586374}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index 41f2f4afd..4d4e80ce 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -470,6 +470,9 @@
 extern const char kEnableResamplingScrollEventsName[];
 extern const char kEnableResamplingScrollEventsDescription[];
 
+extern const char kEnableResourceLoadingHintsName[];
+extern const char kEnableResourceLoadingHintsDescription[];
+
 extern const char kEnableSyncUserConsentSeparateTypeName[];
 extern const char kEnableSyncUserConsentSeparateTypeDescription[];