Add {Get,Set}CohortHint interface.

Export the "cohort hint" getter and setter in the client interfaces
(D-Bus and Binder). The cohort hint is sent to Omaha on every update
check request but can be ignored and/or reset by Omaha on every
response.

Other minor linter fixes to the affected files.

Bug: 31740109
Test: Build with D-Bus and with Binder.

(cherry picked from commit 5b5fa8b412312a41cfd4d7ab475b54d4f730ed2a)

Change-Id: Ieb40c2d4bbe2ce926d54b6348b22f2840d8327db
Reviewed-on: https://chromium-review.googlesource.com/395206
Commit-Ready: Alex Deymo <deymo@chromium.org>
Tested-by: Alex Deymo <deymo@chromium.org>
Reviewed-by: Christopher Book <cbook@chromium.org>
diff --git a/client_library/client_binder.h b/client_library/client_binder.h
index cd857e0..b1b34da 100644
--- a/client_library/client_binder.h
+++ b/client_library/client_binder.h
@@ -53,6 +53,9 @@
                  std::string* out_new_version,
                  int64_t* out_new_size) const override;
 
+  bool SetCohortHint(const std::string& in_cohort_hint) override;
+  bool GetCohortHint(std::string* out_cohort_hint) const override;
+
   bool SetUpdateOverCellularPermission(bool allowed) override;
   bool GetUpdateOverCellularPermission(bool* allowed) const override;