Coverage: add a trace for not_covered_lines per file

Example output:

{
    "build_number": "2",
    "gitHash": "71f5db892ee7ed9fbb573da28ccb906e18ba24a4",
    "key": {
        "arch": "x86_64",
        "compiler": "Clang",
        "configuration": "Coverage",
        "cpu_or_gpu": "CPU",
        "cpu_or_gpu_value": "AVX2",
        "model": "GCE",
        "os": "Ubuntu"
    },
    "results": {
        "dm_DMGpuSupport_h": {
            "coverage": {
                "lines_not_covered": 6,
                "options": {
                    "dir": "dm",
                    "fullname": "dm/DMGpuSupport.h",
                    "source_type": "coverage"
                },
                "percent": 33.33333333333333
            }
        },
        "dm_DMJsonWriter_cpp": {
......

BUG=skia:2430

Review URL: https://codereview.chromium.org/1246043002
1 file changed