Reland "DevTools: Block SIGPROF during CPU throttling"

This is a reland of a5afd47a

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}

Bug: 1027242
Change-Id: I2c5c1e462f9f6f16739e773dda112bacd8b354f9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2014404
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734429}

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

@@ -106,6 +106,9 @@ 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();
+
+  // Run some JS for at least a second to give a chance for the two signals to
+  // overlap.
+  await session.evaluate(`
+    let count = 0;
+    const limit = 1e6;
+    let i = 0;
+    let target = new Date();
+    target.setSeconds(target.getSeconds() + 1);
+    let time = new Date();
+    while (i < limit && time < target) {
+      count += i;
+      i++;
+      time = new Date();
+    }
+    window.count = count;
+  `);
+
+  await dp.Profiler.stop();
+
+  testRunner.completeTest();
+})