report: import <detail> polyfill as string, not Buffer

[brendankenny]

Fixes viewer bundle size inflation introduced by #6593. The details-element-polyfill was accidentally being inlined as a base64-encoded Buffer and then having toString() called on it.

The string version is smaller, and we don't also need Buffer browserified in :)

[brendankenny]

$ bundlesize
 PASS  ./dist/extension/scripts/lighthouse-ext-bundle.js: 361.57kB < maxSize 520kB gzip 
 PASS  ./dist/viewer/src/viewer.js: 58.21kB < maxSize 65kB gzip 

back where they should be.

[paulirish]

budget success!!!