#3636 Makes Aggregator#process use walkInPlatformIndependentOrder

[sub101]

related issue #3636

Motivation:

• At AggregateDepsTask cache miss due to inconsistent order #3636, we fix AggregateDepsTask cache miss due to inconsistent order by walkInPlatformIndependentOrder
• ✅ But on Aggregator#process, it still uses walkTopDown. so I think it's good to replace Aggregator#walkTopDown to walkInPlatformIndependentOrder too for consistency on all platforms

Modification:

• Replace Aggregator#walkTopDown to walkInPlatformIndependentOrder too for consistency
• Add walkInPlatformIndependentOrder unit test

Result:

• Now Aggregator#process works in consistent order on all platforms!

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