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V8 caches FunctionTemplates for a lifetime of a web page for its own internal
reasons, thus it is generally a good idea to cache the template returned by
gin::CreateFunctionTemplate function. Otherwise, repeated method invocations
from JS will create substantial memory leaks.

BUG=463487

Review URL: https://codereview.chromium.org/974183005

Cr-Commit-Position: refs/heads/master@{#321330}