Add a test case for highlight annotation with opacity.

R=thestig@chromium.org

Review URL: https://codereview.chromium.org/2204763002 .
diff --git a/pdfium/annots/annotation_highlight_opacity.pdf b/pdfium/annots/annotation_highlight_opacity.pdf
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diff --git a/pdfium/annots/annotation_highlight_opacity_expected.pdf.0.png b/pdfium/annots/annotation_highlight_opacity_expected.pdf.0.png
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