[DevTools] Only issue Page.viewportChanged event when device emulation is enabled.
[DevTools] Only issue Page.viewportChanged event when device emulation is enabled.

BUG=none

Review URL: https://codereview.chromium.org/362773003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@177378 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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