set_sources_assignment_filter() is considered as a mis-feature
of gn (as it makes it difficult to reason about the BUILD.gn
files individually).

Change BUILD.gn in SwiftShader to check the platform before
adding plaform-specific source files to "sources" variable.
This will allow removing the global sources assignment filter
from Chromium codebase.

Bug: chromium:1018739
Change-Id: Icf357de347b09dccce8c031d84ae7d8ce0ec396b
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/48668Tested-by: Corentin Wallez <cwallez@google.com>
Reviewed-by: Corentin Wallez <cwallez@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>