Hide implementations of ScopedAllowIO/ScopedAllowWait like other scoped thread restrictions objects

Re-uses existing helpers to make these truly empty impls in non-dcheck
builds.

Most importantly, this allows instrumenting these calls with extra
tracing information when debugging without having to rebuild the world
(thread_restrictions.h is implicitly included virtually everywhere).

(and yes they should be outright removed in favor of the new ones but
that's orthogonal to this CL)

R=fdoray@chromium.org

Bug: gab doesn't like to rebuild the world when debugging
Change-Id: I6f7f39a26164c4375fd9b48233236a528b0fa2b5
Reviewed-on: https://chromium-review.googlesource.com/969446
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544340}

base/threading/thread_restrictions.cc

@@ -118,6 +118,13 @@ void ResetThreadRestrictionsForTesting() {
 
 }  // namespace internal
 
+ThreadRestrictions::ScopedAllowIO::ScopedAllowIO()
+    : was_allowed_(SetIOAllowed(true)) {}
+
+ThreadRestrictions::ScopedAllowIO::~ScopedAllowIO() {
+  SetIOAllowed(was_allowed_);
+}
+
 // static
 bool ThreadRestrictions::SetIOAllowed(bool allowed) {
   bool previous_disallowed = g_blocking_disallowed.Get().Get();
@@ -157,6 +164,13 @@ bool ThreadRestrictions::SetWaitAllowed(bool allowed) {
   return !previous_disallowed;
 }
 
+ThreadRestrictions::ScopedAllowWait::ScopedAllowWait()
+    : was_allowed_(SetWaitAllowed(true)) {}
+
+ThreadRestrictions::ScopedAllowWait::~ScopedAllowWait() {
+  SetWaitAllowed(was_allowed_);
+}
+
 }  // namespace base
 
 #endif  // DCHECK_IS_ON()

base/threading/thread_restrictions.h

@@ -359,11 +359,13 @@ class BASE_EXPORT ThreadRestrictions {
   // DEPRECATED. Use ScopedAllowBlocking(ForTesting).
   class BASE_EXPORT ScopedAllowIO {
    public:
-    ScopedAllowIO() { previous_value_ = SetIOAllowed(true); }
-    ~ScopedAllowIO() { SetIOAllowed(previous_value_); }
+    ScopedAllowIO() EMPTY_BODY_IF_DCHECK_IS_OFF;
+    ~ScopedAllowIO() EMPTY_BODY_IF_DCHECK_IS_OFF;
+
    private:
-    // Whether IO is allowed when the ScopedAllowIO was constructed.
-    bool previous_value_;
+#if DCHECK_IS_ON()
+    const bool was_allowed_;
+#endif
 
     DISALLOW_COPY_AND_ASSIGN(ScopedAllowIO);
   };
@@ -470,12 +472,13 @@ class BASE_EXPORT ThreadRestrictions {
   // DEPRECATED. Use ScopedAllowBaseSyncPrimitives.
   class BASE_EXPORT ScopedAllowWait {
    public:
-    ScopedAllowWait() { previous_value_ = SetWaitAllowed(true); }
-    ~ScopedAllowWait() { SetWaitAllowed(previous_value_); }
+    ScopedAllowWait() EMPTY_BODY_IF_DCHECK_IS_OFF;
+    ~ScopedAllowWait() EMPTY_BODY_IF_DCHECK_IS_OFF;
+
    private:
-    // Whether singleton use is allowed when the ScopedAllowWait was
-    // constructed.
-    bool previous_value_;
+#if DCHECK_IS_ON()
+    const bool was_allowed_;
+#endif
 
     DISALLOW_COPY_AND_ASSIGN(ScopedAllowWait);
   };