Windows: Look into LIGHTHOUSE_CHROMIUM_PATH variable too.

[XhmikosR]

Fixes #1472.

[paulirish]

sg. shall we add it for Mac too just so it's straightforward to reason about?

if this is set, then we should definitely use it rather than any sniffed location.

[XhmikosR]

I guess it makes sense to take into consideration the variable. Although it should be documented somewhere.

I need this for #1280 and noticed that only on Linux the env var was taken into consideration.

[brendankenny]

+1 for mac support and having it override any other choice (unless you use --select-chrome in addition to setting the variable).

win32 will need the priorities treatment that the other platforms get, and mac will need to set process.env.LIGHTHOUSE_CHROMIUM_PATH as highest priority in the same way linux does

[XhmikosR]

@brendankenny: what does that mean? That this won't be merged currently?

[brendankenny]

actually, I guess this does handle with and without --select-chrome since without a sort step the value in LIGHTHOUSE_CHROMIUM_PATH will win, as you would expect.

We can handle macs and documenting the variable in the future.

LGTM