Web UI Polymer 2: Turn on by default on Chrome OS

Polymer 2 is currently on by default on all platforms except for
Chrome OS. The automated tests are now all passing with Polymer 2 on
Chrome OS, with the exception of OOBE/Login related tests. This CL
turns on Polymer 2 by default on Chrome OS except for the OOBE page,
so that Web UI pages using Polymer can be tested to see if there are
any additional issues that have not been detected by the tests.

See https://crrev.com/c/1414590 for the addition of the
WebUIPolymer2Exceptions flag, which ensures that Polymer 1 will still
be used for the OOBE page after this CL lands.

Bug: 738611
Change-Id: Ic103a94f2b1e2283596daa9b54ac2d60aa210ba4
Reviewed-on: https://chromium-review.googlesource.com/c/1447026
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628572}

content/public/common/content_features.cc

@@ -674,14 +674,8 @@ const char kWebXrRenderPathParamValueSharedBuffer[] = "SharedBuffer";
 // Makes all WebUI that uses Polymer use 2.x version.
 // TODO(dpapad): Remove this once Polymer 2 migration is done,
 // https://crbug.com/738611.
-const base::Feature kWebUIPolymer2 {
-  "WebUIPolymer2",
-#if !defined(OS_CHROMEOS)
-      base::FEATURE_ENABLED_BY_DEFAULT
-#else
-      base::FEATURE_DISABLED_BY_DEFAULT
-#endif  // !defined(OS_CHROMEOS)
-};
+const base::Feature kWebUIPolymer2{"WebUIPolymer2",
+                                   base::FEATURE_ENABLED_BY_DEFAULT};
 #endif  // !defined(OS_ANDROID)
 
 #if defined(OS_CHROMEOS)