Remove V8PerIsolateData::templateMap()

This CL replaces V8PerIsolateData::templateMap() with V8PerIsolateData::existingDOMTemplate()/setDOMTemplate(). Then this CL removes V8PerIsolateData::templateMap().

BUG=341032

Review URL: https://codereview.chromium.org/195823005

git-svn-id: svn://svn.chromium.org/blink/trunk@169002 bbb929c8-8fbe-4397-9dbb-9b2b20218538