Update debug_renderer script to pass --disable-features=SpareRendererForSitePerProcess.

The spare renderer confuses the script, which expects to attach to the first
(and only) renderer.

Some complexity is required to ensure that other --disable-features flags
passed by the user take effect, since our command line parsing code won't
work as intended if we have two different --disable-features flags.

Change-Id: I2058fc2d5f9d8afa6bd4c5cd673b502458f476f4
Reviewed-on: https://chromium-review.googlesource.com/c/1416612
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Reviewed-by: Robert Ma <robertma@chromium.org>
Auto-Submit: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623780}
1 file changed