Reset and then populate frontend features during display initialize

Overwritten features were never reset even if a display was
terminated. On platforms that reuse displays for all tests
in the end2end suite, overridden feature would leak into
subsequent tests causing unexpected failure.

Bug: angleproject:8235
Change-Id: I1b359bc762a2bca8db4e4dbc7a587604e5bd6a5b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4643453
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: mohan maiya <m.maiya@samsung.com>

include/platform/Feature.h

@@ -182,6 +182,7 @@ struct FeatureSetBase
     FeatureMap members = FeatureMap();
 
   public:
+    void reset();
     void overrideFeatures(const std::vector<std::string> &featureNames, bool enabled);
     void populateFeatureList(FeatureList *features) const;