[TaskScheduler]: Document Process::WaitForExitWithTimeout as not considered blocking.

Original CL intention was:
Use ScopedBlockingCallWithBaseSyncPrimitives to annotate sync primitives.

Migrate AssertBaseSyncPrimitivesAllowed() to
ScopedBlockingCallWithBaseSyncPrimitives in base::Process.

Bug: 903957, 905788
Change-Id: Ief5072d7e9b9feaf1ee7b817add26545bf80b78a
Reviewed-on: https://chromium-review.googlesource.com/c/1324418Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608768}

base/process/process_posix.cc

@@ -337,6 +337,9 @@ bool Process::WaitForExit(int* exit_code) const {
 }
 
 bool Process::WaitForExitWithTimeout(TimeDelta timeout, int* exit_code) const {
+  // Intentionally avoid instantiating ScopedBlockingCallWithBaseSyncPrimitives.
+  // In some cases, this function waits on a child Process doing CPU work.
+  // http://crbug.com/905788
   if (!timeout.is_zero())
     internal::AssertBaseSyncPrimitivesAllowed();
 

base/process/process_win.cc

@@ -181,6 +181,9 @@ bool Process::WaitForExit(int* exit_code) const {
 }
 
 bool Process::WaitForExitWithTimeout(TimeDelta timeout, int* exit_code) const {
+  // Intentionally avoid instantiating ScopedBlockingCallWithBaseSyncPrimitives.
+  // In some cases, this function waits on a child Process doing CPU work.
+  // http://crbug.com/905788
   if (!timeout.is_zero())
     internal::AssertBaseSyncPrimitivesAllowed();