[workspace] Make UISourceCode a SafeContentProvider

This CL implements 'requestContentData' for UISourceCode and also
changes the internal implementation to use the safer ContentData instead
of a DeferredContent.

The CL has to add some helper methods. In particular the
'checkContentUpdated' method checks whether content changed. To this
end it decodes any base64 encoded content into binary string. This
is somewhat ok as long as this unsafely decoded content is only used
for comparison but not actually show it to the user or store it.

The CL also updates the UISourceCode unit tests. It used 'setContent'
with non-base64 content but set the 'isBase64' flag. With the new
implementation this causes issues for several reasons: 1) The
test doesn't set up a proper mime type so the ContentData assumes
it's binary data that can't be decoded. So attempting to access the
ContentData as text throws an error. 2) The strings used in the
test are not actually valid base64 so decoding would fail in any
case.

R=kimanh@chromium.org, leese@chromium.org

Bug: 342529015
Change-Id: Ie71d30d2e4fbaeb0e4bd3e64ea06a73123d1ba13
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/5570126
Reviewed-by: Philip Pfaffe <pfaffe@chromium.org>
Commit-Queue: Simon Zünd <szuend@chromium.org>