[ES6 modules] enable dynamic import by default

Intent-to-ship thread w/ 3 lgtms:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/ut-Mr0jt5X8/Q8B4F3wxBQAJ

Bug: 711706
Change-Id: Ib28083a8473fb74a1f6222c4c6cf84d9a810564a
Reviewed-on: https://chromium-review.googlesource.com/691394Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Kouhei Ueno <kouhei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505288}

content/public/common/content_features.cc

@@ -176,7 +176,7 @@ const base::Feature kModuleScripts{"ModuleScripts",
 
 // ES6 Modules dynamic imports.
 const base::Feature kModuleScriptsDynamicImport{
-    "ModuleScriptsDynamicImport", base::FEATURE_DISABLED_BY_DEFAULT};
+    "ModuleScriptsDynamicImport", base::FEATURE_ENABLED_BY_DEFAULT};
 
 // Resource fetch optimizations for workers. See crbug.com/443374
 const base::Feature kOffMainThreadFetch{"OffMainThreadFetch",