PDF: Reduce / fix ScopedUnsupportedFeature usage.

In PDFiumEngine, instead of putting ScopedUnsupportedFeature in
functions that call PDFium document loading code, and hope their scope
is sufficiently, use the narrowest possible scope, which is guaranteed
to be just right.

Also add a ScopedUnsupportedFeature instatiation for form loading, as
PDFium can also report unsupported features during that process. This
avoids a potential NOTREACHED() in ScopedUnsupportedFeature code.

Bug: 1071223
Change-Id: Iaf79ff2bae7b41a92f7bb69585a7fa9eb6278ac5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2151480
Reviewed-by: Daniel Hosseinian <dhoss@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759471}