Add new StringHashMap container (bug 5878 part 1, r=ds).

public/amtl/am-hashmap.h

@@ -131,6 +131,10 @@ class HashMap : public AllocPolicy
     return table_.add(i);
   }
 
+  void clear() {
+    table_.clear();
+  }
+
   size_t estimateMemoryUse() const {
     return table_.estimateMemoryUse();
   }

public/amtl/am-hashtable.h

@@ -60,6 +60,10 @@ namespace detail {
       destruct();
       hash_ = kRemovedHash;
     }
+    void setFree() {
+      destruct();
+      hash_ = kFreeHash;
+    }
     void initialize() {
       hash_ = kFreeHash;
     }
@@ -432,6 +436,14 @@ class HashTable : public AllocPolicy
     return true;
   }
 
+  void clear() {
+    for (size_t i = 0; i < capacity_; i++) {
+      table_[i].setFree();
+    }
+    ndeleted_ = 0;
+    nelements_ = 0;
+  }
+
   size_t estimateMemoryUse() const {
     return sizeof(Entry) * capacity_;
   }