Reduce minified size with some small tweaks
Reduce minified size with some small tweaks

This patch:
- Creates local aliases for global functions. These can have their names
  shortened by the minifier.
- Assigns some strings to variables. These, also, can be shortened.
- Replaces a short circuit return with an else block. This if/else can be
  replaced with a ternary by the minifier.

I'm not sure how the minified source that is checked in was generated. I
tested these changes using `uglify-js@3.4.9`. This tool produced slightly
larger baseline source than is checked in, at 787 bytes (without the preamble
comment). This patch reduces that to 696 bytes, an ~11% reduction.

GoogleChromeLabs/first-input-delay