arc: Switch ArcCustomTabs to disabled by default

Currently ArcCustomTabs is enabled by default with no easy way (ie no
longer listed in chrome://flags) for the user to disable, this shouldn't
be an issue but due to stability concerns it would be better to assume a
safer approach, this CL does that by marking such feature as
DISABLED_BY_DEFAULT.

Bug: 1065124
Test: Build.
Change-Id: I0682b05629bc5892ad04871f244f3e3b7119dcf7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2506151
Auto-Submit: David Jacobo <djacobo@chromium.org>
Reviewed-by: Yusuke Sato <yusukes@chromium.org>
Commit-Queue: David Jacobo <djacobo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821903}

components/arc/arc_features.cc

@@ -20,7 +20,7 @@ const base::Feature kCleanArcDataOnRegularToChildTransitionFeature{
 
 // Controls experimental Custom Tabs feature for ARC.
 const base::Feature kCustomTabsExperimentFeature{
-    "ArcCustomTabsExperiment", base::FEATURE_ENABLED_BY_DEFAULT};
+    "ArcCustomTabsExperiment", base::FEATURE_DISABLED_BY_DEFAULT};
 
 // Controls whether ARC handles child->regular account transition.
 const base::Feature kEnableChildToRegularTransitionFeature{