Update pdfium/annots/annotation_underline_expected.pdf.0.png with actual underline annotation.

Also, add a test case for underline annotation with opacity.

R=thestig@chromium.org

Review URL: https://codereview.chromium.org/2209483002 .
diff --git a/pdfium/annots/annotation_underline_expected.pdf.0.png b/pdfium/annots/annotation_underline_expected.pdf.0.png
index 2a75c63..aff0e0b 100644
--- a/pdfium/annots/annotation_underline_expected.pdf.0.png
+++ b/pdfium/annots/annotation_underline_expected.pdf.0.png
Binary files differ
diff --git a/pdfium/annots/annotation_underline_opacity.pdf b/pdfium/annots/annotation_underline_opacity.pdf
new file mode 100644
index 0000000..0f0cd5f
--- /dev/null
+++ b/pdfium/annots/annotation_underline_opacity.pdf
Binary files differ
diff --git a/pdfium/annots/annotation_underline_opacity_expected.pdf.0.png b/pdfium/annots/annotation_underline_opacity_expected.pdf.0.png
new file mode 100644
index 0000000..a867d9b
--- /dev/null
+++ b/pdfium/annots/annotation_underline_opacity_expected.pdf.0.png
Binary files differ