CrOS: Remove ScopedAllowIO in SupportsPSI

Checking path existence in procfs is fast, it's not necessary to use
ScopedAllowIO.

Bug: 1166221
Change-Id: I60ec3f5865239ba426f3064ce5a05d6688320de8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2631807
Commit-Queue: Brian Geffon <bgeffon@chromium.org>
Reviewed-by: Brian Geffon <bgeffon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844105}

chromeos/memory/pressure/pressure.cc

@@ -82,8 +82,8 @@ uint64_t GetReservedMemoryKB() {
 }
 
 bool SupportsPSI() {
-  static bool supports_psi =
-      base::PathExists(base::FilePath("/proc/pressure/"));
+  // Checking path existence in procfs is fast.
+  static bool supports_psi = access("/proc/pressure/", F_OK) == 0;
   return supports_psi;
 }