refactor: replace USE_X11 with OS_LINUX for sharing code with Ozone

[noploso]

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

[MonnetalX]

🔍 PR Link Analysis

🔗 Ranked Links

#1 these Wayland patches

🔍 The linked comparison shows a series of patches that replace USE_X11 with OS_LINUX conditionals and remove X11-specific code from Ozone builds, which is the same refactoring approach this PR is taking to enable Wayland support.

#2 going away

🔍 The pull request replaces USE_X11 with OS_LINUX to enable sharing code with Ozone, which is a necessary step for supporting Wayland on Linux, as Wayland is now the default display server on Ubuntu and provides better rendering performance on embedded devices compared to X11.

#3 commit guidelines

🔍 The pull request aims to replace the USE_X11 preprocessor macro with OS_LINUX to enable sharing code with Ozone, which is a first step toward supporting Wayland in Electron.

#4 https://github.com/electron/electron/blob/master/CONTRIBUTING.md

🔍 The CONTRIBUTING.md outlines the project's guidelines for contributions, including the release cadence of three latest major versions every 8 weeks, which is relevant to understanding the scope and timing of the Wayland support patches being upstreamed.

#5 chromium#1085700

🔍 The webpage content is empty, so no relevant information can be extracted.

#6 these chromium changes

🔍 The webpage describes Gerrit Code Review, a web-based code review system for Git repositories, which is relevant to the PR's goal of sharing code between X11 and Wayland by replacing USE_X11 with OS_LINUX.