Improve process output watcher's handling of multi-byte UTF8 characters

Improve process output watcher's handling of multi-byte UTF8 characters

Speciffically, handle case when a non ASCII UTF8 character is read
partially from the process output. Instead of reporting read character
bytes imediatelly, cache them until the rest of the character is read
from the process output.

BUG=278340
TEST=in crosh (see comment #1 in the bug):
$python
>>> print(u"\u20ac" * 10000)
Verify a series of EURO signs is displayed (without "unknown" characters)

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=270799

[tbarzic, 2014-05-01 16:00:23]

can you please take a look?

[tbarzic, 2014-05-06 21:38:19]

On 2014/05/01 16:00:23, tbarzic wrote:
> can you please take a look?

ping

[rginda, 2014-05-14 20:34:33]

lgtm

Sorry for the delay, I missed the mail about this one.

[tbarzic, 2014-05-14 20:36:48]

On 2014/05/14 20:34:33, rginda wrote:
> lgtm
> 
> Sorry for the delay, I missed the mail about this one.

np.. I have similar experiences with chromium-review.googlesource.com :)

[tbarzic, 2014-05-14 20:41:42]

The CQ bit was checked by tbarzic@chromium.org

[commit-bot: I haz the power, 2014-05-14 20:42:06]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/tbarzic@chromium.org/261743002/100001

[commit-bot: I haz the power, 2014-05-15 00:18:37]

FYI, CQ is re-trying this CL (attempt #1).
Please consider checking whether the failures are real,
and report flakes to chrome-troopers@google.com.
The failing builders are:
  linux_chromium_chromeos_clang_dbg on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/linux_chromium_chrome...)

[commit-bot: I haz the power, 2014-05-15 01:43:00]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-05-15 01:43:00]

Try jobs failed on following builders:
  linux_chromium_chromeos_clang_dbg on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/linux_chromium_chrome...)

[tbarzic, 2014-05-15 17:45:23]

The CQ bit was checked by tbarzic@chromium.org

[commit-bot: I haz the power, 2014-05-15 17:47:26]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/tbarzic@chromium.org/261743002/120001

[commit-bot: I haz the power, 2014-05-15 20:34:40]

Change committed as 270799