[unseasoned-pdf] Add GetPluginImageData() in PdfViewPluginBase.

Adds GetPluginImageData() in PdfViewPluginBase which can return the
the plugin-specific image data buffer in `Image` format.

This method will hide the Pepper-only member `pepper_image_data_`
behind it while the PaintReadyRect constructor is called in DoPaint(),
and allows DoPaint() to be migrated into PdfViewPluginBase.

Bug: 1140629
Change-Id: I004bc6072d81214df0ee39ce1b51b0ac750e8c1e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2659247


Commit-Queue: Hui Yingst <nigi@chromium.org>
Reviewed-by: K. Moon <kmoon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#848818}