Force enable ShadowDOM V0 and Custom Element V0 for PDF extension.

This patch is a preparation for Shadow DOM V0, Custom Element V0 (CEv0) and HTML Imports removal.
Intent to Deprecate and Remove: http://bit.ly/2RGHe3x
Though we soon make these APIs only available for test by turning the json5 feature flag
from status: "stable" to status: "test", there is browser tests/chrome internal components
that use them.

PDF extension is a one of them and this patch reenable the APIs for it.
We should migration it using new APIs.

This is leftover from previous patch: crrev.com/c/1405955 (only enable HTML Import)
that's because Polymer1 uses SDv0 and CEv0.

on crrev.com/c/148130

Bug: 924031
TEST: browser_tests --gtest_filter=*PDFExtensionTest* --disable-features=WebUIPolymer2
Change-Id: If7569e629d2c3b1472181d2a641209807b8fe748
Reviewed-on: https://chromium-review.googlesource.com/c/1493687Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Yoichi Osato <yoichio@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636367}

extensions/renderer/dispatcher.cc

@@ -893,8 +893,11 @@ void Dispatcher::OnActivateExtension(const std::string& extension_id) {
   // use the old web APIs.
   // After completion of the migration, we should remove this.
   // See crbug.com/924031 for detail.
-  if (extension_id == extension_misc::kPdfExtensionId)
+  if (extension_id == extension_misc::kPdfExtensionId) {
+    blink::WebRuntimeFeatures::EnableShadowDOMV0(true);
+    blink::WebRuntimeFeatures::EnableCustomElementsV0(true);
     blink::WebRuntimeFeatures::EnableHTMLImports(true);
+  }
 
   InitOriginPermissions(extension);