Unify Sampler implementation across platforms using signals to suspend thread

pthreads seem to be fully supported on Linux and Android which allows to remove many #ifdefs.

Also OS::Sleep(interval_) is used to pause sampling thread on all platforms. It makes no sense to send signal once 900mks to compensate 100mks delay on signal delivery as the signals would be delivered once 900mks.

BUG=None

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=14278

[Sven Panne, 2013-04-16 11:28:59]

LGTM. At least if the sampler still works on Android... ;-) More seriously: I am
not 100% sure if the pthreads implementation on Android is close enough for this
to work, but we'll see. ;-)

[yurys, 2013-04-16 12:02:51]

On 2013/04/16 11:28:59, Sven Panne wrote:
> LGTM. At least if the sampler still works on Android... ;-) More seriously: I
am
> not 100% sure if the pthreads implementation on Android is close enough for
this
> to work, but we'll see. ;-)

Quick test showed similar results on Nexus 7 but dispersion is very high so it
is hard to tell from that test how close the implementations are.

[yurys, 2013-04-16 12:06:57]

Committed patchset #3 manually as r14278 (presubmit successful).