Revert "Wire up ShillSandboxing flag to be enabled by default"

This reverts commit e034d163.

Reason for revert: We want to change shill sandboxing back to
default-disabled rather than default-enabled. We will still roll out
sandboxing to 50% of users in M70.

Original change's description:
> Wire up ShillSandboxing flag to be enabled by default
>
> In a previous CL:1173260 I meant to wire up this feature flag to be
> enabled by default. Somehow I mistakenly landed the code with the
> FEATURE_DISABLED_BY_DEFAULT flag instead.
>
> Bug: 649417
> Test: Same as CL:1173260
> Change-Id: I2154749addb0539be9a2ef53c368d80dc10bee41
> Reviewed-on: https://chromium-review.googlesource.com/1190644
> Reviewed-by: Mattias Nissler <mnissler@chromium.org>
> Commit-Queue: Micah Morton <mortonm@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#586444}

TBR=stevenjb@chromium.org,mnissler@chromium.org,mortonm@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 649417
Change-Id: Ice783f6bced9828e1ae8fb966f00f5dc32aff3f2
Reviewed-on: https://chromium-review.googlesource.com/1251326Reviewed-by: Mattias Nissler <mnissler@chromium.org>
Commit-Queue: Micah Morton <mortonm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595474}

chrome/common/chrome_features.cc

@@ -649,7 +649,7 @@ const base::Feature kUsbguard{"USBGuard", base::FEATURE_ENABLED_BY_DEFAULT};
 
 // Enable running shill in a minijail sandbox on Chrome OS.
 const base::Feature kShillSandboxing{"ShillSandboxing",
-                                     base::FEATURE_ENABLED_BY_DEFAULT};
+                                     base::FEATURE_DISABLED_BY_DEFAULT};
 #endif  // defined(OS_CHROMEOS)
 
 // Enable showing a tab-modal dialog while a Web Authentication API request is