refactor: replace USE_X11 with OS_LINUX for sharing code with Ozone

[c4rtune]

Description of Change

This pull-request is a small first step in trying to bring support for Wayland to Electron. I'm planning to follow up on this with more pull-requests in an effort to upstream these Wayland patches.

However, those patches will need some adjustments in order to make them compatible with the upcoming Chromium changes (chromium#1085700) which will soon be merged into Electron (#25488). In particular, it should be taken into consideration that the USE_X11 and USE_OZONE build flags are no longer mutually exclusive.

Also, USE_X11 is going away so it might be desirable as a long term goal to remove it from Electron too and replace it with the Ozone equivalents.

With that in mind, this pull-request replaces a few instances of #if defined(USE_X11) with #if defined(OS_LINUX) where it was identified that the guarded code could be shared with Ozone.

The more meaningful changes will come in the upcoming pull-requests but I'll be waiting for these chromium changes to be pulled into Electron first (#24984, #25488).

Checklist

• PR description included and stakeholders cc'd (@zcbenz @hedgepigdaniel)
• npm test passes
• PR title follows semantic commit guidelines

Release Notes

Notes: none