Update ChromeContextMenuPopulatorTest#testReadLater.

testReadLater intends to verify that the read_later context menu should
not display on touching a non-http scheme URL link. In the test the URL
comes from LINK_URL which is a normal http URL, instead, use an "about"
scheme for testing, it's an acceptable scheme but non-http scheme, more
realistic for testing.

Bug: 396563950
Test: tools/autotest.py -C out/Default ChromeContextMenuPopulatorTest
Change-Id: I2eaf6ac5252d495490c2a5b977367411eef042c4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6275471
Reviewed-by: Peter Conn <peconn@chromium.org>
Reviewed-by: Stefano Duo <stefanoduo@google.com>
Commit-Queue: Xiao Ma <xiaom@google.com>
Cr-Commit-Position: refs/heads/main@{#1421316}