DevTools: PageAgent: do not use ScriptResource::script as it changes state.
DevTools: PageAgent: do not use ScriptResource::script as it changes state.

BUG=418536

Review URL: https://codereview.chromium.org/633073002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@183658 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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