Support setting 'better' and using it in expectations.

tools/perf_expectations/perf_expectations.json

 {"linux-release-64/sizes/chrome-bss/bss": {"reva": 105312, "revb": 105336, "type": "absolute", "improve": 595081, "regress": 657756, "sha1": "c1e34d47"},
  "linux-release-64/sizes/chrome-data/data": {"reva": 91119, "revb": 91326, "type": "absolute", "improve": 2309480, "regress": 2557742, "sha1": "fd84a9d2"},
- "linux-release-64/sizes/chrome-si/initializers": {"reva": 107131, "revb": 107134, "type": "absolute", "improve": 290, "regress": 293, "tolerance": 0, "sha1": "2d95f23c"},
+ "linux-release-64/sizes/chrome-si/initializers": {"reva": 107131, "revb": 107134, "type": "absolute", "better": "lower", "improve": 293, "regress": 293, "tolerance": 0, "sha1": "0e693f17"},
  "linux-release-64/sizes/chrome-text/text": {"reva": 104463, "revb": 104463, "type": "absolute", "improve": 60823945, "regress": 67905522, "sha1": "6887f46b"},
  "linux-release-64/sizes/chrome.pak/chrome.pak": {"reva": 93584, "revb": 93660, "type": "absolute", "improve": 2893618, "regress": 3230516, "sha1": "ddcb764c"},
  "linux-release-64/sizes/chrome/chrome": {"reva": 99734, "revb": 100028, "type": "absolute", "improve": 78759552, "regress": 87104926, "sha1": "48bce0e1"},
  "linux-release-hardy/dhtml/commit_charge/cc": {"reva": 63231, "revb": 64325, "improve": -6947, "regress": 25528, "sha1": "0e22e67a"},
  "linux-release-hardy/dhtml/processes/proc_": {"improve": 0, "regress": 0, "sha1": "696fcb0f"},
  "linux-release-hardy/dhtml/times/t": {"reva": 99393, "revb": 100034, "improve": -68, "regress": 21, "sha1": "9c13cabb"},
  "linux-release-hardy/dhtml/total_byte_b/IO_b": {"reva": 85983, "revb": 88209, "improve": 4132, "regress": 6013, "sha1": "b7cb3bfb"},
  "linux-release-hardy/dhtml/total_byte_r/IO_r": {"reva": 71820, "revb": 73091, "improve": -43, "regress": -38, "sha1": "9a4533d7"},
  "linux-release-hardy/dhtml/total_op_b/IO_op_b": {"reva": 70127, "revb": 76230, "improve": 26261, "regress": 40613, "sha1": "1658213e"},
  "linux-release-hardy/dhtml/total_op_r/IO_op_r": {"reva": 80853, "revb": 81211, "improve": 60, "regress": 698, "sha1": "17890f30"},
@@ skipping 27 matching lines @@
  "linux-release-lowmem/sunspider/total/t": {"reva": 81217, "revb": 81884, "improve": -645, "regress": 722, "sha1": "35f98911"},
  "linux-release-lowmem/v8_benchmark/score/score": {"reva": 81069, "revb": 81884, "improve": 1196, "regress": -2933, "sha1": "9e618ede"},
  "linux-release/dhtml/times/t": {"reva": 99393, "revb": 100034, "improve": -21, "regress": 145, "sha1": "6cb344a1"},
  "linux-release/dromaeo_jslib/jslib/score": {"reva": 90010, "revb": 104572, "improve": -146, "regress": -201, "sha1": "f9b75f63"},
  "linux-release/intl1/times/t": {"reva": 104868, "revb": 105328, "improve": 196, "regress": 558, "sha1": "b48b1979"},
  "linux-release/intl2/times/t": {"reva": 104868, "revb": 105328, "improve": -23, "regress": 239, "sha1": "0dd1d8e2"},
  "linux-release/morejs/times/t": {"reva": 105307, "revb": 105595, "improve": 32, "regress": 79, "sha1": "f0b0f169"},
  "linux-release/moz/times/t": {"reva": 99393, "revb": 100034, "improve": -40, "regress": 172, "sha1": "2cf4039f"},
  "linux-release/sizes/chrome-bss/bss": {"reva": 105307, "revb": 105336, "type": "absolute", "improve": 362825, "regress": 401052, "sha1": "ac6bb167"},
  "linux-release/sizes/chrome-data/data": {"reva": 98413, "revb": 98423, "type": "absolute", "improve": 1187520, "regress": 1325781, "sha1": "6c95adae"},
- "linux-release/sizes/chrome-si/initializers": {"reva": 107131, "revb": 107134, "type": "absolute", "improve": 293, "regress": 296, "tolerance": 0, "sha1": "3af6c71e"},
+ "linux-release/sizes/chrome-si/initializers": {"reva": 107131, "revb": 107134, "type": "absolute", "better": "lower", "improve": 296, "regress": 296, "tolerance": 0, "sha1": "38e78ff9"},
  "linux-release/sizes/chrome-text/text": {"reva": 91010, "revb": 101959, "type": "absolute", "improve": 50917816, "regress": 59260180, "sha1": "d50aeb06"},
  "linux-release/sizes/chrome.pak/chrome.pak": {"reva": 93584, "revb": 93660, "type": "absolute", "improve": 2893618, "regress": 3230516, "sha1": "d1047e17"},
  "linux-release/sizes/chrome/chrome": {"reva": 99734, "revb": 100028, "type": "absolute", "improve": 68631100, "regress": 75912682, "sha1": "62b2e7be"},
  "linux-release/startup/times/t": {"improve": -15, "regress": 15, "sha1": "c8f06f81"},
  "mac-release-10.5/dhtml/times/t": {"reva": 102949, "revb": 104239, "improve": -153, "regress": 218, "sha1": "4f4b0e51"},
  "mac-release-10.5/intl1/times/t": {"reva": 105307, "revb": 106585, "improve": -114, "regress": 282, "sha1": "ac0ca5cd"},
  "mac-release-10.5/intl2/commit_charge/cc": {"reva": 79662, "revb": 81211, "improve": -46238, "regress": 55911, "sha1": "4c75e106"},
  "mac-release-10.5/intl2/processes/proc_": {"improve": 0, "regress": 0, "sha1": "a1a53594"},
  "mac-release-10.5/intl2/times/t": {"reva": 80750, "revb": 81208, "improve": -1772, "regress": 1586, "sha1": "3b491046"},
  "mac-release-10.5/intl2/vm_rss_final_b/vm_rss_f_b": {"reva": 99507, "revb": 100009, "improve": -7881, "regress": 7858, "sha1": "db007fa1"},
@@ skipping 49 matching lines @@
  "xp-release-single-core/intl2/times/t": {"reva": 105307, "revb": 105734, "improve": 69, "regress": 1117, "sha1": "8aa60255"},
  "xp-release-single-core/morejs/times/t": {"reva": 104308, "revb": 104575, "improve": -32, "regress": 485, "sha1": "e63333ad"},
  "xp-release-single-core/moz/times/t": {"reva": 75758, "revb": 76221, "improve": -1705, "regress": 3304, "sha1": "b615a416"},
  "xp-release-single-core/startup/times/t": {"improve": -9, "regress": 9, "sha1": "b1bf03da"},
  "xp-release/sizes/chrome.dll/chrome.dll": {"reva": 100820, "revb": 100961, "type": "absolute", "improve": 35191526, "regress": 38926004, "sha1": "94524a77"},
  "xp-release/sizes/chrome.exe/chrome.exe": {"reva": 81814, "revb": 82805, "type": "absolute", "improve": 1057453, "regress": 1180570, "sha1": "24fe20f6"},
  "xp-release/sizes/mini_installer.exe/mini_installer.exe": {"reva": 106747, "revb": 106771, "type": "absolute", "improve": 19560089, "regress": 21758823, "sha1": "f29a1a3c"},
  "xp-release/sizes/setup.exe/setup.exe": {"reva": 81852, "revb": 82805, "type": "absolute", "improve": 1127275, "regress": 1258522, "sha1": "13bbca59"},
  "load": true
 }