Avoid memory allocation on base::Feature lookup

FeatureList::IsFeatureEnabled and FeatureList::GetAssociatedFieldTrial
call std::map<std::string, _>::find() with a const char* argument. By
default, this results in a conversion to an std::string, hence
incurring a memory allocation.

C++14 adds a new std::map::find() override to support map lookups by
types other than the key when a transparent comparator is specified.
This allows, for example, performing lookups in an
std::map<std::string, _> using a const char*. This CL changes the
declartion of FeatureList::overrides_ to support these allocation-free
lookups.

Bug: 960618

Change-Id: I59c8d2c16e86a3a6ca458b4047c445b1373df05d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1600083
Commit-Queue: Collin Baker <collinbaker@chromium.org>
Reviewed-by: Albert J. Wong <ajwong@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657544}

base/feature_list.h

@@ -5,6 +5,7 @@
 #ifndef BASE_FEATURE_LIST_H_
 #define BASE_FEATURE_LIST_H_
 
+#include <functional>
 #include <map>
 #include <memory>
 #include <string>
@@ -287,7 +288,7 @@ class BASE_EXPORT FeatureList {
 
   // Map from feature name to an OverrideEntry struct for the feature, if it
   // exists.
-  std::map<std::string, OverrideEntry> overrides_;
+  std::map<std::string, OverrideEntry, std::less<>> overrides_;
 
   // Locked map that keeps track of seen features, to ensure a single feature is
   // only defined once. This verification is only done in builds with DCHECKs