SkPDF: draw{Image,Bitmap} always serializes early

Before this change, the PDFCanon held a map from BitmapKeys
to SkImages for de-duping bitmaps.  Even if the PDFDocument
serialized images early, the Canon still held a ref to that
image inside the map.  With this change, the Canon holds a
single map from BitmapKeys to PDFObjects.  Now, Images are
only held by the PDFObject, which the document serializes
and drops early.

This change also:

-   Moves SkBitmapKey into its own header (for possible
    reuse); it now can operate with images as well as
    bitmaps.

-   Creates SkImageBitmap, which wraps a pointer to a bitmap
    or an image and abstracts out some common tasks so that
    drawBitmap and drawImage behave the same.

-   Modifies SkPDFCreateBitmapObject to take and return a
    sk_sp<T>, not a T*.

-   Refactors SkPDFDevice::internalDrawImage to use bitmaps
    or images (via a SkImageBitmap).

-   Turns on pre-serialization of all images.

BUG=skia:5087

GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1829693002

Review URL: https://codereview.chromium.org/1829693002
11 files changed