Revert "Convert GN arg clang_unsafe_buffers_paths to a list."

This reverts commit 66618d28e1599ff9e8c7f1339c32525684afec46.

Reason for revert: broke cronet

Original change's description:
> Convert GN arg clang_unsafe_buffers_paths to a list.
>
> Then, prior to compilation, process all listed files into a
> single generated file to be used by the plugin. In this manner,
> rolling a dependent product will automatically update the
> suppressed paths without the need to keep a top-level file
> consistent. This also prevents the plugin from needing to open
> several unsafe_buffers_paths files for each compilation unit.
>
> Complications are that the paths in the dependent product
> unsafe_buffers_paths file will need to be re-written relative
> to the location in the top-level product's tree. In addition.
> there needs to be a new entry for the implicit '+' rule at
> each dependent product's top-level directory.
>
> This is taken care of by a new python script. Pre-process comments
> and blank lines here as well, so that we may later save a few
> cycles on each compilation unit by not doing this in the plugin
> (in a future CL once this sticks). The usual GN path rebasing needs
> to take place in order to find the files. Most of the work involves
> this issue.
>
> Convert chromium to import PDFium's suppression in this manner.
>
> There will need to be a similar change when this rolls into
> PDFium to avoid breakage, converting its //.gn file to specify
> a list (of one) for the unsafe_buffers_paths argument.
>
> Bug: 389830173
> Change-Id: I24be31bfd3f6a75f74b197870cd5f0431fa8199f
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6167583
> Reviewed-by: Hans Wennborg <hans@chromium.org>
> Commit-Queue: Tom Sepez <tsepez@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1406292}

Bug: 389830173
Change-Id: I835e32d62a47ea6e20dd2b20a12d3adc419fa436
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6175885
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Owners-Override: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Auto-Submit: Tom Sepez <tsepez@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1406404}