[iOS] Clean up xcode_extra_attributes usages in XcodeWriter.

https://chromium-review.googlesource.com/c/567619 assigns values to
xcode_extra_attributes in BUILD files, so clean up the hard-coded
values in GN.

Bug: 741147, 709289
Change-Id: I564fa63f258fe3ade29dbeeb7e08b1876d7abcd8
Reviewed-on: https://chromium-review.googlesource.com/614864
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#494800}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: bafca0f82098a1e000bbca31a47b8b43834f55c4
1 file changed
tree: 84cc5f6a856fe4fd3dd7d99af0e53a86f2e41388
  1. tools/