Some of the new features in Review Board 3.0 regressed support for some

slightly older browsers. There was a non-ES6 JavaScript file using

const, and an expectation that Blob was a valid type.
The const has been replaced with a var statement, and the Blob is

now checked as a valid type before use. This restores IE9 and some

mobile support.

Tested that IE9 no longer broke on page load.