Add PrintTo for string16

Add PrintTo for string16

This is required by googletest to print a readable output when failing to compare two string16's (ref: http://goo.gl/jALbO)

Without this the output of a failed comparison could be:

Value of: ElideEmail(UTF8ToUTF16(testcases[i].input), font, font.GetStringWidth(expected_output))
  Actual: { 97, 116, 34, 64, 64, 64, 64, 64, 64, 64, 64, 64, 46, 46, 46, 64, 64, 46, 8230, 64, 109, 97, 100, 110, 101, 115, 115, 46, 99, 111, 109 }
Expected: expected_output
Which is: { 97, 116, 34, 64, 64, 64, 64, 64, 64, 64, 64, 64, 46, 46, 46, 64, 64, 46, 64, 46, 8230, 64, 109, 97, 110, 101, 115, 115, 46, 99, 111, 109 }

With this patch the same error is now properly formatted:

Value of: ElideEmail(UTF8ToUTF16(testcases[i].input), font, font.GetStringWidth(expected_output))
  Actual: at"@@@@@@@@@...@@.…@madness.com
Expected: expected_output
Which is: at"@@@@@@@@@...@@.@.…@maness.com

BUG=
TEST=unit_tests


Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=125642

[gab, 2012-03-08 14:48:36]

PTAL

[brettw, 2012-03-08 15:30:22]

lgtm

[commit-bot: I haz the power, 2012-03-08 15:35:03]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/gab@chromium.org/9641017/1

[commit-bot: I haz the power, 2012-03-08 18:34:52]

Change committed as 125642