Revert "DevTools: Block SIGPROF during CPU throttling"

This reverts commit a5afd47a.

Reason for revert: BUG:1044528

Original change's description:
> DevTools: Block SIGPROF during CPU throttling
> 
> The V8 CPU profiler schedules SIGPROF signals. It is very likely that
> the signal handler for these will run while the throttling signal
> handler is already running. V8 can't properly read the stack when
> there is already a signal handler present.
> 
> Fix this by using sa_mask to block V8's SIGPROFs while the
> throttling handler is running. This makes more sense overall as well.
> Our CPU samples would be skewed by whichever stacks we encounter
> during throttling pauses.
> 
> Bug: 1027242
> Change-Id: I80e856ce1e7b4744a9af7e36c2022b6d8610cf60
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2012280
> Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
> Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
> Commit-Queue: Peter Marshall <petermarshall@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#733592}

TBR=skyostil@chromium.org,sigurds@chromium.org,petermarshall@chromium.org

Change-Id: Ic4c9d8c8616fd506bb34b006d27334cba3c96cf4
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1027242
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2012963Reviewed-by: Olga Sharonova <olka@chromium.org>
Commit-Queue: Olga Sharonova <olka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733926}

third_party/blink/renderer/platform/scheduler/common/thread_cpu_throttler.cc

@@ -106,9 +106,6 @@ void ThreadCPUThrottler::ThrottlingThread::InstallSignalHandler() {
   struct sigaction sa;
   sa.sa_handler = &HandleSignal;
   sigemptyset(&sa.sa_mask);
-  // Block SIGPROF while our handler is running so that the V8 CPU profiler
-  // doesn't try to sample the stack while our signal handler is active.
-  sigaddset(&sa.sa_mask, SIGPROF);
   sa.sa_flags = SA_RESTART;
   signal_handler_installed_ =
       (sigaction(SIGUSR2, &sa, &old_signal_handler_) == 0);

third_party/blink/web_tests/inspector-protocol/cpu-profiler/record-cpu-profile-with-cpu-throttling-expected.txt

-Test that the profiler can record a profile with cpu throttling enabled.
-

third_party/blink/web_tests/inspector-protocol/cpu-profiler/record-cpu-profile-with-cpu-throttling.js

-(async function(testRunner) {
-  const {page, session, dp} = await testRunner.startBlank(
-      'Test that the profiler can record a profile with cpu throttling enabled.');
-
-  await dp.Emulation.setCPUThrottlingRate({rate: 4});
-  await dp.Profiler.enable();
-  await dp.Profiler.start();
-
-  await session.evaluate(`
-    let count = 0;
-    for (let i = 0; i < 1e7; i++) {
-      count += i;
-    }
-    window.count = count;
-  `);
-
-  await dp.Profiler.stop();
-
-  testRunner.completeTest();
-})