Revert of Revert "Remove CpuMonitor and related, unused, code." (patchset #1 id:1 of https://codereview.webrtc.org/1287913004/ )

Reason for revert:
(retrying with my webrtc account...)
The reason for reverting is: Re-landing the change that removes the CpuMonitor class after having fixed the build issue in Chromium..

Original issue's description:
> Revert "Remove CpuMonitor and related, unused, code."
>
> This reverts commit 1a24012680f25440aa1d117373df2af14cdc2fc1.
>
> TBR=tommi@webrtc.org,pthatcher@webrtc.org
> BUG=
>
> This breaks
> http://build.chromium.org/p/chromium.webrtc.fyi/builders/Linux/builds/20148/steps/compile/logs/stdio
>
> Committed: a472e968c9

TBR=pthatcher@webrtc.org,guoweis@webrtc.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=

Review URL: https://codereview.webrtc.org/1290033005

Cr-Commit-Position: refs/heads/master@{#9733}

webrtc/base/base_tests.gyp

@@ -15,7 +15,6 @@
         'unittest_main.cc',
         # Also use this as a convenient dumping ground for misc files that are
         # included by multiple targets below.
-        'fakecpumonitor.h',
         'fakenetwork.h',
         'fakesslidentity.h',
         'faketaskrunner.h',
@@ -58,7 +57,6 @@
           'bytebuffer_unittest.cc',
           'byteorder_unittest.cc',
           'callback_unittest.cc',
-          'cpumonitor_unittest.cc',
           'crc32_unittest.cc',
           'criticalsection_unittest.cc',
           'event_tracer_unittest.cc',