Strangely, this isn't happening on the main chromium.perf waterfall on the exact same hardware configuration.

The perf JSON output contains lots of histograms that I wouldn't expect. Here's an example piece of output:

"""
"http://edition.cnn.com": {"std": 0.0, "name": "ForInPrepare:count_min", "page_id": 13, "improvement_direction": "down", "important": false, "values": [113], "units": "count", "type": "list_of_scalar_values"}, "http://www.msn.com/ar-ae": {"std": 0.0, "name": "ForInPrepare:count_min", "page_id": 17, "improvement_direction": "down", "important": false, "values": [23], "units": "count", "type": "list_of_scalar_values"}, "https://www.youtube.com/watch?v=_kZsOISarzg": {"std": 0.0, "name": "ForInPrepare:count_min", "page_id": 2, "improvement_direction": "down", "important": false, "values": [7], "units": "count", "type": "list_of_scalar_values"}, "http://www.twitter.com/taylorswift13": {"std": 0.0, "name": "ForInPrepare:count_min", "page_id": 10, "improvement_direction": "down", "important": false, "values": [64], "units": "count", "type": "list_of_scalar_values"}, "http://pollouer.muc/Speedometer/CustomRunner.html?backbone": {"std": 0.0, "name": "ForInPrepare:count_min", "page_id": 31, "improvement_direction": "down", "important": false, "values": [8], "units": "count", "type": "list_of_scalar_values"}, "http://www.sina.com.cn": {"std": 0.0, "name": "ForInPrepare:count_min", "page_id": 20, "improvement_direction": "down", "important": false, "values": [13], "units": "count", "type": "list_of_scalar_values"}, "summary": {"std": 0.0, "name": "ForInPrepare:count_min", "improvement_direction": "down", "important": false, "values": [4, 4, 7, 10, 18, 145, 559, 21, 64, 24, 51, 113, 97, 25, 8, 23, 26, 13, 14, 8, 28, 5, 26, 16, 1, 8, 8, 4], "units": "count", "type": "list_of_scalar_values"}, "https://www.linkedin.com/m/": {"std": 0.0, "name": "ForInPrepare:count_min", "page_id": 16, "improvement_direction": "down", "important": false, "values": [8], "units": "count", "type": "list_of_scalar_values"}, "https://www.google.de/search?q=v8": {"std": 0.0, "name": "ForInPrepare:count_min", "page_id": 0, "improvement_direction": "down", "important": false, "values": [4], "units": "count", "type": "list_of_scalar_values"}, "http://pollouer.muc/Speedometer/CustomRunner.html?jquery": {"std": 0.0, "name": "ForInPrepare:count_min", "page_id": 30, "improvement_direction": "down", "important": false, "values": [8], "units": "count", "type": "list_of_scalar_values"}}, "API_ObjectTemplate_NewInstance:duration_sum": {"http://www.msn.com/ar-ae": {"std": 0.0, "name": "API_ObjectTemplate_NewInstance:duration_sum", "page_id": 17, "improvement_direction": "down", "important": false, "values": [0.224], "units": "ms", "type": "list_of_scalar_values"}, "http://www.twitter.com/taylorswift13": {"std": 0.0, "name": "API_ObjectTemplate_NewInstance:duration_sum", "page_id": 10, "improvement_direction": "down", "important": false, "values": [0.042], "units": "ms", "type": "list_of_scalar_values"}, "http://www.pinterest.com/categories/popular": {"std": 0.0, "name": "API_ObjectTemplate_NewInstance:duration_sum", "page_id": 19, "improvement_direction": "down", "important": false, "values": [0.044], "units": "ms", "type": "list_of_scalar_values"},
"""

What?! It looks like it's creating metrics for every trace event name. I'm not at all sure why it would be doing this.