Delete backspace-for-back RuntimeFeature.

third_party/WebKit/public/platform/WebFeature.h

 #ifndef WebFeature_h
 #define WebFeature_h
 
 namespace blink {
 // ============ Definition for WebFeature used for UseCounter ===============
 //
 // Do not change assigned numbers of existing items: add new features
 // to the end of the list.
 //
 // A WebFeature conceptually represents some particular web-exposed API
@@ skipping 613 matching lines @@
   kNetInfoDownlinkMax = 947,
   kNetInfoOnChange = 948,
   kNetInfoOnTypeChange = 949,
   kV8Window_Alert_Method = 950,
   kV8Window_Confirm_Method = 951,
   kV8Window_Prompt_Method = 952,
   kV8Window_Print_Method = 953,
   kV8Window_RequestIdleCallback_Method = 954,
   kFlexboxPercentagePaddingVertical = 955,
   kFlexboxPercentageMarginVertical = 956,
-  kBackspaceNavigatedBack = 957,
-  kBackspaceNavigatedBackAfterFormInteraction = 958,
   kCSPSourceWildcardWouldMatchExactHost = 959,
   kCredentialManagerGet = 960,
   kCredentialManagerGetMediationOptional = 961,
   kCredentialManagerGetMediationSilent = 962,
   kCredentialManagerStore = 963,
   kCredentialManagerRequireUserMediation = 964,
   // The above items are available in M47 branch.
 
   kBlockableMixedContentInSubframeBlocked = 966,
   kAddEventListenerThirdArgumentIsObject = 967,
@@ skipping 919 matching lines @@
   // Add new features immediately above this line. Don't change assigned
   // numbers of any item, and don't reuse removed slots.
   // Also, run update_use_counter_feature_enum.py in
   // chromium/src/tools/metrics/histograms/ to update the UMA mapping.
   kNumberOfFeatures,  // This enum value must be last.
 };
 
 }  // namespace blink
 
 #endif  // WebFeature_h