Fix IsUsingShelfAutoDim

Currently --enable-dim-shelf flag has the opposite effect (dimming is
disabled when the flag is present, and vice versa). This is due to a
spurious negation in the IsUsingShelfAutoDim implementation.

Bug: 996006
Test: manual
Change-Id: I44340bbacb2a998f57176bb7405c39cf4309380d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1899088Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Auto-Submit: Vladislav Kaznacheev <kaznacheev@chromium.org>
Commit-Queue: Vladislav Kaznacheev <kaznacheev@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712670}

ash/public/cpp/ash_switches.cc

@@ -134,7 +134,7 @@ bool IsUsingViewsLock() {
 }
 
 bool IsUsingShelfAutoDim() {
-  return !base::CommandLine::ForCurrentProcess()->HasSwitch(kEnableDimShelf);
+  return base::CommandLine::ForCurrentProcess()->HasSwitch(kEnableDimShelf);
 }
 
 }  // namespace switches