Canvas assignments with lots of text cut off the PDF.js viewer

**Describe the bug**
When a Canvas user creates an assignment with a lot of text and embeds Hypothesis, the Hypothesis window gets cut off at the bottom of the screen (i.e., Canvas will not allow scrolling all of the way to the bottom of the window).

**To Reproduce**
Steps to reproduce the behavior:
1. Create a new Canvas assignment.
2. Add a large amount of text to the text field (for example, I added 25 lines of text, consisting of a period and a return on each line).
3) Embed follow our normal instructions for embedding Hypothesis and adding a URL or a PDF via a Picker.

**Expected behavior**
I would expect the text from the text field to be on top of the Hypothesis window. When scrolling down the screen, I would expect to be able to see the entire Hypothesis window after scroll past all of the text.

Instead, once I've scrolled as far as I can scroll, I only see about 1/3 to 1/2 of the embedded Hypothesis window.

You can see an example in our Canvas instance, class = "Support 102", assignment = "assignment text".

**Screenshots**
If applicable, add screenshots to help explain your problem.

**Desktop (please complete the following information):**
- OS: iOS
- Browser Chrome
- Version 79