Make files created in base::PlatformFile readable by grp and oth on ChromeOS

Create files readable by grp and oth in base::PlatformFile for posix.

Main motivation for this is webkit's createFile.
While creating files with permission 0600 is not an issue on other platforms,
on ChromeOs FileBrowser may access external filesystem, so it makes sense to
create files with permission 0644.
Further more creating files with mode 0600 on ChromeOS external fielsystem may
cause some problems with zip file support (see bug).

Note this is M16 release blocker...

BUG=chromium-os:22263
TEST=linux_chromeos trybots
     verified copying files in FileBrowser on ChromeOs creates files with permission 644

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=110740

[tbarzic, 2011-11-17 04:18:45]

hi guys,
could you please have a look at this...

[rvargas (doing something else), 2011-11-17 22:43:55]

LGTM.

[ericu, 2011-11-17 23:12:09]

LGTM.

[commit-bot: I haz the power, 2011-11-18 04:32:05]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/tbarzic@chromium.org/8590020/1

[commit-bot: I haz the power, 2011-11-18 04:32:08]

Presubmit check for 8590020-1 failed and returned exit status 1.

Running presubmit commit checks ...

** Presubmit ERRORS **
Missing LGTM from an OWNER for: base/platform_file_posix.cc

[darin (slow to review), 2011-11-18 07:27:52]

LGTM

[commit-bot: I haz the power, 2011-11-18 17:48:19]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/tbarzic@chromium.org/8590020/1

[commit-bot: I haz the power, 2011-11-18 19:47:43]

Try job failure for 8590020-1 (retry) (retry) on linux_rel for step "ui_tests".
It's a second try, previously, step "ui_tests" failed.
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=linux_rel&...