[base] Remove base::KEEP_LAST_OF_DUPES from base::flat_tree

This change removes base::KEEP_LAST_OF_DUPES and thus FlatContainerDupes
from the flat associative containers. This change leads to making the
interface of flat_tree cleaner, simplifying its implementation and
aligning it closer to what is currently proposed for standardisation
as std::flat_set [1] and std::flat_map [2].

base::JSONParser, the only customer of KEEP_LAST_OF_DUPES, is modified
to keep the same semantics.

[1] https://wg21.link/p1222
[2] https://wg21.link/p0429

TBR=rockot,avi,thakis,kmackay

Bug: 682254
Change-Id: I2189331df969c42d0de7cc4924bed35abdaffc52
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1816408
Commit-Queue: Jan Wilken Dörrie <[email protected]>
Reviewed-by: Avi Drissman <[email protected]>
Reviewed-by: Nico Weber <[email protected]>
Reviewed-by: Kenneth MacKay <[email protected]>
Reviewed-by: Daniel Cheng <[email protected]>
Cr-Commit-Position: refs/heads/master@{#698928}
diff --git a/chrome/browser/resources_util.cc b/chrome/browser/resources_util.cc
index e2bbf23a9..a5701567 100644
--- a/chrome/browser/resources_util.cc
+++ b/chrome/browser/resources_util.cc
@@ -56,7 +56,7 @@
     }
 #endif
 
-    id_map_ = StringIntMap(std::move(storage), base::KEEP_FIRST_OF_DUPES);
+    id_map_ = StringIntMap(std::move(storage));
   }
 
   int GetId(const std::string& resource_name) const {