Guarantee that every PDF metadata field is trimmed of whitespace
characters, so users of the metadata do not have to worry about checking
themselves.

Consequently, change PDFiumEngine::GetMetadataByField() to
PDFiumEngine::GetTrimmedMetadataByField().

Bug: 93619
Change-Id: Ie51c7099d147e50e4e507e43fcca3a17fbba183a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2609655
Commit-Queue: Daniel Hosseinian <dhoss@chromium.org>
Reviewed-by: K. Moon <kmoon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841712}