[GENERIC viewer] Fix the background color of the findInput when the search term is not found

When a search term isn't found, the background color of the findInput is supposed to change (to red). This is currently not working as intended, because the CSS rule is not being applied correctly. (It seems that this broke in PR 2208.)

This patch also changes the background color to match the one used in the native Firefox findbar, since the old color seemed a bit too pink.
Prevent a "TypeError: pdfViewer is null" when the viewer loads (PR 5413 followup)

Currently if you manage to e.g. open the console (with <kbd>Ctrl</kbd>+<kbd>Shift</kbd>+<kbd>K</kbd>) before the viewer is initialized, the following will be printed in the console: `TypeError: pdfViewer is null`.
This doesn't cause any actual errors, but nevertheless it seems like something we should avoid.

Followup to PR 5413.
Fixes typo in webgl.js
Merge pull request #5492 from jsundn/master

Fixes typo in webgl.js
Merge pull request #5488 from Snuffleupagus/presentationMode-no-zoom-followup

Prevent a "TypeError: pdfViewer is null" when the viewer loads (PR 5413 followup)
Merge pull request #5465 from Snuffleupagus/findbar-notFound-color

[GENERIC viewer] Fix the background color of the findInput when the search term is not found
Merge pull request #1 from mozilla/master

Sync to master