Disable TerminalSystemApp by dfeault for M83

Terminal will not be on by default in M83.  This CL is created
to be merged in M83 beta, then TerminalSystemApp will be
re-enabled on trunk in a followup CL.

Bug: 1016190
Change-Id: Idee2ccc19402a69fa33ef21506e48c6761f18a3b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2138651Reviewed-by: Jason Lin <lxj@google.com>
Commit-Queue: Jason Lin <lxj@google.com>
Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Auto-Submit: Joel Hockey <joelhockey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756903}

chrome/common/chrome_features.cc

@@ -179,7 +179,7 @@ const base::Feature kPrintServerUi{"PrintServerUi",
 // OS_CHROMEOS, but this flag must be defined for all platforms since
 // it is required for SystemWebApp tests.
 const base::Feature kTerminalSystemApp{"TerminalSystemApp",
-                                       base::FEATURE_ENABLED_BY_DEFAULT};
+                                       base::FEATURE_DISABLED_BY_DEFAULT};
 
 // Use the Terminal System App legacy settings page.
 const base::Feature kTerminalSystemAppLegacySettings{