Print Preview: Add flag to enable the alternate UI.

This is the first step allowing to develop a componetized (with Polymer)
Print Preview UI, without affecting the existing UI during development.

Bug: 773928
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: Ia40f7de733d5694953175098549420f10eb1a49b
Reviewed-on: https://chromium-review.googlesource.com/717697
Commit-Queue: Demetrios Papadopoulos <[email protected]>
Reviewed-by: Lei Zhang <[email protected]>
Cr-Commit-Position: refs/heads/master@{#511344}
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index cf80fef..9fcde67 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -2853,6 +2853,11 @@
      FEATURE_VALUE_TYPE(
          features::kDisplayPersistenceToggleInPermissionPrompts)},
 #endif
+#if BUILDFLAG(ENABLE_PRINT_PREVIEW)
+    {"enable-new-print-preview", flag_descriptions::kEnableNewPrintPreview,
+     flag_descriptions::kEnableNewPrintPreviewDescription, kOsDesktop,
+     FEATURE_VALUE_TYPE(features::kNewPrintPreview)},
+#endif
 #if BUILDFLAG(ENABLE_PRINT_PREVIEW) && !defined(OS_WIN) && !defined(OS_MACOSX)
     {"print-pdf-as-image", flag_descriptions::kPrintPdfAsImageName,
      flag_descriptions::kPrintPdfAsImageDescription, kOsDesktop,