Restore previous dribbling-subprocess-output code for non-Windows. (Windows code adds buffering when used on POSIX adding uncomfortable delays in output).  Windows code retained for use on Windows only.
Restore previous dribbling-subprocess-output code for non-Windows.
(Windows code adds buffering when used on POSIX adding uncomfortable
delays in output).  Windows code retained for use on Windows only.

Add command line flag --dont-clear-coverage-data to facilitate tester use.  Workflow:
1. compile for coverage
2. run chrome and do whatever
3. Run coverage script by hand; e.g.
  tools/code_coverage/coverage_posix.py --directory out/Debug --genhtml out/index.html --dont-clear-coverage-data 

The coverage_posix.py tool otherwise deletes all coverage data when it
starts since it assumes it only wants coverage data generated by it's
subprocesses.


BUG=none
TEST=coverage bots stay green; Anantha happier.


Review URL: http://codereview.chromium.org/2870054

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@52713 0039d316-1c4b-4281-b951-d872f2087c98

adobe/chromium,ropik/chromium,Crystalnix/house-of-life-chromium,adobe/chromium,ropik/chromium,gavinp/chromium,yitian134/chromium,ropik/chromium,Crystalnix/house-of-life-chromium,Crystalnix/house-of-life-chromium,gavinp/chromium,adobe/chromium,adobe/chromium,yitian134/chromium,yitian134/chromium,ropik/chromium,yitian134/chromium,gavinp/chromium,yitian134/chromium,yitian134/chromium,Crystalnix/house-of-life-chromium,Crystalnix/house-of-life-chromium,yitian134/chromium,gavinp/chromium,Crystalnix/house-of-life-chromium,gavinp/chromium,Crystalnix/house-of-life-chromium,adobe/chromium,ropik/chromium,yitian134/chromium,gavinp/chromium,Crystalnix/house-of-life-chromium,ropik/chromium,adobe/chromium,adobe/chromium,ropik/chromium,Crystalnix/house-of-life-chromium,ropik/chromium,gavinp/chromium,ropik/chromium,Crystalnix/house-of-life-chromium,adobe/chromium,adobe/chromium,adobe/chromium,yitian134/chromium,gavinp/chromium,yitian134/chromium,gavinp/chromium,Crystalnix/house-of-life-chromium,adobe/chromium,gavinp/chromium