[iOS][wk2] Use ImageDocument to display subframe PDFs

[iOS][wk2] Use ImageDocument to display subframe PDFs
https://bugs.webkit.org/show_bug.cgi?id=133944
<rdar://problem/17205983>

Reviewed by Dan Bates.

* dom/DOMImplementation.cpp:
(WebCore::DOMImplementation::createDocument):
Make a ImageDocument to show our subframe PDF, if the setting is enabled.

* html/ImageDocument.cpp:
(WebCore::ImageDocument::finishedParsing):
(WebCore::ImageDocument::createDocumentStructure):
Set the ResourceResponse on the ImageElement upon creation, instead of
when the load finishes. This way, when the internal CachedImage creates
its Image, it can have the correct MIME type for the response, and can
make a PDFDocumentImage if necessary.

We never noticed this before because CachedImage falls back to making a
BitmapImage, and that was the only case that was normally used.

Throw an explicit white background behind PDF ImageDocuments; PDFDocumentImage
paints with a transparent background, but when used as an ImageDocument
it seems reasonable to assume that there should be a white page background
(similar to what PDFPlugin and friends do).

* loader/DocumentWriter.cpp:
(WebCore::DocumentWriter::createDocument):
Don't make a PDFDocument if we want to use an ImageDocument for this subframe PDF.

* page/Settings.in:

* platform/MIMETypeRegistry.cpp:
(WebCore::MIMETypeRegistry::isPDFMIMEType):
* platform/MIMETypeRegistry.h:
Expose the already implemented pdfMIMETypes table just like all the rest.

* WebProcess/WebPage/WebPage.cpp:
(WebKit::WebPage::updatePreferences):
Always use ImageDocument for subframe PDFs in WebKit2 on iOS.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@170091 268f45cc-cd09-0410-ab3c-d52691b4dbfc