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The recently added 'HashTraits::PeekInType' type allows us to remove
HashMap<RefPtr..> template specialization without loosing the efficiency
and without bringing any significant changes and complexity to the HashMap
class.

To get rid of 'RefPtrHashMap' a new 'RefPtrValuePeeker' type was created
and used as 'HashTraits::PeekInType' for RefPtr. The 'RefPtrValuePeeker'
class can be constructed either from RefPtr, PassRefPtr or a plain pointer,
then it behaves like a plain pointer itself within HashTable methods.

The proposed change brings the following benefits:
1) Removes tons of duplicated code
2) Allows avoiding ref-count churn also at HashSet<RefPtr>

Review URL: https://codereview.chromium.org/184233006

git-svn-id: svn://svn.chromium.org/blink/trunk@168593 bbb929c8-8fbe-4397-9dbb-9b2b20218538