Linux, Android: fix call to setpriority(2).

Linux, Android: fix call to setpriority(2).

setpriority(), when used with PRIO_PROCESS, should affect the whole
process (i.e. thread group). On Linux the system call only affects one
thread (this bug is documented in
http://man7.org/linux/man-pages/man2/getpriority.2.html) and thus lets
the caller specify a tid.

This CL changes the call to setpriority(PRIO_PROCESS, tid, ...) to
setpriority(PRIO_PROCESS, 0, ...), which is strictly equivalent, albeit
slightly more correct.

This makes filtering system calls in our sandbox easier, as the current thread
id is not known to the BPF filter.

BUG=399473
R=mdempsky@chromium.org, willchan@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=286964

[jln (very slow on Chromium), 2014-08-01 00:01:56]

Matthew: could you please take a look?

[mdempsky, 2014-08-01 00:05:32]

https://chromiumcodereview.appspot.com/434803003/diff/1/base/threading/platfo...
File base/threading/platform_thread_android.cc (right):

https://chromiumcodereview.appspot.com/434803003/diff/1/base/threading/platfo...
base/threading/platform_thread_android.cc:74: if (setpriority(PRIO_PROCESS, 0,
kNiceSetting))
Can SetThreadPriority() be called by a different thread?  Should there be an
assert to check that handle.id_ == gettid(), or maybe even just use

    handle.id_ == gettid() ? 0 : handle.id_

with a comment that we prefer to use 0 if possible because it allows tighter
sandbox policies?

(Same comment on the _linux.cc file.)

[willchan no longer on Chromium, 2014-08-01 00:09:18]

Let me know if you substantially change anything, but otherwise, this LGTM from
a base/OWNERS perspective. I leave the details to mdempsky.

[jln (very slow on Chromium), 2014-08-01 00:39:53]

Matthew: please take another look! (and send to CQ if you're happy).

https://chromiumcodereview.appspot.com/434803003/diff/1/base/threading/platfo...
File base/threading/platform_thread_android.cc (right):

https://chromiumcodereview.appspot.com/434803003/diff/1/base/threading/platfo...
base/threading/platform_thread_android.cc:74: if (setpriority(PRIO_PROCESS, 0,
kNiceSetting))
Ooch. I was a little hasty on this one indeed, thanks.

[mdempsky, 2014-08-01 00:42:06]

lgtm

[commit-bot: I haz the power, 2014-08-01 00:44:59]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/jln@chromium.org/434803003/20001

[jln (very slow on Chromium), 2014-08-01 07:51:43]

Committed patchset #2 manually as r286964 (presubmit successful).