Split `leftover_args` if items have multiple flags.

Pintpoint passes passthrough_args as a space-separated string to allow
for multiple flags. This string needs to be split into a list of
individual arguments. When processing the shards map, these arguments
should be passed as passthrough_args.

Fixed: 382037844
Change-Id: I1c2ade2743817bcd9553e75e1d4634ae931c2bf6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6065163
Reviewed-by: John Chen <johnchen@chromium.org>
Commit-Queue: Farid (Mojtaba) Faridzad <faridzad@google.com>
Cr-Commit-Position: refs/heads/main@{#1391988}