handle pdf type in ExternalNavigationDelegateImpl.
On android, not all the pdf link are handled by the DownloadManagerService.
If a pdf link triggers a new tab to be opened, that is handled by ExternalNavigationHandler.
The ExternalNavigationHandler should use the same logic in DownloadManagerService to open the link in PdfViewer.
This change :
1. modifies the StartActivity() and StartAcvitityIfNeeded() call to handle pdf.
Both functions will now modify the intent if it can be handled by the PdfViewer.
We need to modify both calls because one is used for incognito and the other is for non-incognito mode.
2. Move the logic of pdf handling from DownloadManagerService to ExternalNavigationDelegateImpl for code reuse purpose.
BUG=478304
Review URL: https://codereview.chromium.org/
1125993002
Cr-Commit-Position: refs/heads/master@{#328593}