Reland "Create a feature flag for PDF portfolio"

This is a reland of commit 747748b35537c1bd952ab266114334354785a125.
This CL removes the extra line in
tools/metrics/histograms/enums.xml.

Original change's description:
> Create a feature flag for PDF portfolio
>
> Creates a feature flag `pdf-portfolio` which will be an indicator that
> portfolio feature is enabled in the PDF viewer. Currently this flag is
> a dummy flag that has not been linked to the actual feature yet.
>
> Bug: 177188
> Change-Id: Ief14cbdad785bad405bf402d3cb57b22314a07c1
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4651500
> Reviewed-by: Lei Zhang <[email protected]>
> Commit-Queue: Nigi <[email protected]>
> Cr-Commit-Position: refs/heads/main@{#1164297}

Bug: 177188
Change-Id: I5c37624ad198ec0a6f7efdfbbeb4d2f4c928fb08
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4661237
Reviewed-by: Lei Zhang <[email protected]>
Commit-Queue: Nigi <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1167665}
diff --git a/pdf/pdf_features.h b/pdf/pdf_features.h
index 9a42c08..a9c6dfd 100644
--- a/pdf/pdf_features.h
+++ b/pdf/pdf_features.h
@@ -16,6 +16,7 @@
 BASE_DECLARE_FEATURE(kAccessiblePDFForm);
 BASE_DECLARE_FEATURE(kPdfIncrementalLoading);
 BASE_DECLARE_FEATURE(kPdfPartialLoading);
+BASE_DECLARE_FEATURE(kPdfPortfolio);
 BASE_DECLARE_FEATURE(kPdfUseSkiaRenderer);
 BASE_DECLARE_FEATURE(kPdfXfaSupport);