[Cronet] Add ios_app_bundle_id_prefix gn arg to cr_cronet.py on iOS.

This is needed for proper dev signing of binaries executed on the device.
The alternative https://crrev.com/c/2232816 would use `setup-gn.py` script,
which is more invasive.

The default ios_app_bundle_id_prefix is "chromium.org" which is most
likely NOT going to work for anybody.

Pass the "bundle-id-prefix" argument with company-specific prefix value,
for example:

```
cr_cronet.py gn -i --bundle-id-prefix="com.google"
```

Bug: 1091876
Change-Id: Id8357be2eabf5fb51b9229694c5476cfbac1bf62
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2241186
Reviewed-by: Zhongyi Shi <zhongyi@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Misha Efimov <mef@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781861}