refactor: replace CHILD_PLUGIN with CHILD_EMBEDDER_FIRST on macOS
Chromium removed upstream support for child plugin processes without
library validation in https://crbug.com/461717105, which we patched
back via feat_restore_macos_child_plugin_process.patch.
Chromium's CHILD_EMBEDDER_FIRST mechanism already provides the right
extensibility point for this: values > CHILD_EMBEDDER_FIRST are reserved
for embedders and resolved via ContentBrowserClient::GetChildProcessSuffix().
Chrome itself uses this pattern for its Alerts helper process.
This commit replaces the Chromium patch with an Electron-native
implementation.
Co-authored-by: trop[bot] <37223003+trop[bot]@users.noreply.github.com>
Co-authored-by: Shelley Vohr <shelley.vohr@gmail.com>
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