When all source files are instrumented, the coverage data for url/origin_unittest.cc is:
{u'count': 609, u'covered': 603, u'percent': 99.01477832512316}

When only instrument url/origin_unittest.cc, the coverage data is:
{u'count': 609, u'covered': 602, u'percent': 98.85057471264368}

the number differs by 1 line, should look into this case to understand what exactly caused the difference.