check for getEntriesByName in performance before calling

[jroebu14]

Resolves #159

Fixes the web-vitals package breaking applications in Opera Mini.

getEntriesByName is not supported in some version of Opera.

First reported here bbc/simorgh#9153

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[philipwalton]

Thanks! Can you sign the CLA so I can merge? Otherwise I'm happy to just make the change myself. Either way :)

[jroebu14]

@googlebot I signed it!

[philipwalton]

Thanks!

[jroebu14]

You're welcome!