[cobrowse] Do not assume turn id is populated for aim threads

There are some scenarios where aim has a thread ID but no turn ID or
a non-standard turn ID. The most common case for this is share links
generated for a thread where "mstk" is replaced by "smstk". When
refreshing the page after loading one of these links, the URL
generation logic for the embedded page would expect a turn ID to be
available when instead there was nullopt, the bad optional access
would crash the browser. This patch makes it acceptable for a
thread to be maintained without a turn ID. For these threads, the
next input will produce an mstk.

(cherry picked from commit e11e38529ee69912dc3e4cd9a011aa5701b88930)

Bug: 502651552
Change-Id: I6af6a35ef47b90ee9f3741014e73541ee87b2163
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7766129
Reviewed-by: Duncan Mercer <mercerd@google.com>
Commit-Queue: Matthew Jones <mdjones@chromium.org>
Cr-Original-Commit-Position: refs/heads/main@{#1615900}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7782653
Bot-Commit: rubber-stamper@appspot.gserviceaccount.com <rubber-stamper@appspot.gserviceaccount.com>
Owners-Override: Srinivas Sista <srinivassista@chromium.org>
Cr-Commit-Position: refs/branch-heads/7727_111@{#3}
Cr-Branched-From: d1ead9fc1b005c1458ac6bfb40d3f6b9404ec4f7-refs/branch-heads/7727@{#3353}
Cr-Branched-From: ce01102937348db7b88c8a4257ee4b3ac702eb1a-refs/heads/main@{#1596535}