Enable XFA forms in the PDF plugin.

This CL enables the XFA forms functionality in both GYP and GN builds.

BUG=chromium:62400

Review-Url: https://codereview.chromium.org/1950123002
Cr-Original-Commit-Position: refs/heads/master@{#393870}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 08e9434b6f4938517755815da37752b0cd906be7

common.gypi

@@ -887,10 +887,10 @@
           'enable_pdf%': 1,
           'pdf_enable_v8%': 1,
         }],
-        ['chromeos==1 or OS=="android" or OS=="ios" or (embedded==1 and chromecast==0)', {
+        ['OS=="android" or OS=="ios" or (embedded==1 and chromecast==0)', {
           'pdf_enable_xfa%': 0,
         }, {
-          'pdf_enable_xfa%': 0, # Reverted pending crbug.com/596373
+          'pdf_enable_xfa%': 1,
         }],
 
         ['chromeos==1 or OS=="android" or OS=="ios" or desktop_linux==1', {