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It simply forwards the flag from v8::FunctionCallbackInfo and allows
the function handler that accept gin::Arguments argument to distinguish
between normal function invocation and call as an object construction.

BUG=

Review URL: https://codereview.chromium.org/401823002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@284123 0039d316-1c4b-4281-b951-d872f2087c98