Extensions: Fix ExtensionResource::GetFilePath for network paths

On Windows, `NormalizeFilePath` fails if the path doesn't start with a
drive letter (e.g. a network path) or if it exceeds `MAX_PATH` characters
in length. Fall back to `MakeAbsoluteFilePath` and proceed if the path
exists.

Bug: 410059474
Change-Id: I4f9e74e5fe741f8c6db471046164919be1380d3c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6604976
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Michael Maltsev (R&D) <michaelm@island.io>
Reviewed-by: Devlin Cronin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1467650}