Merge to XFA: Change the JS app.viewerType() return value to "pdfium"

BUG=pdfium:181
TBR=tsepez@chromium.org

Review URL: https://codereview.chromium.org/1331633002 .

(cherry picked from commit 4bc64cabc561d5df211ed02dc75c85e41fa91312)

Review URL: https://codereview.chromium.org/1327523007 .
5 files changed