[DNS] Enable cobalt_browsertests on Linux with is_starboard=True

[linxinan-chops]

Enable cobalt_browsertests on Linux with starboard.

Also plumbed ContentMainRunnerImplBrowserTest as a demo.

Test:

out/linux-x64x11_devel/cobalt_browsertests --single-process-tests --gtest_filter=*ContentMainRunnerImplBrowserTest.*  --no-sandbox --single-process --no-zygote --ozone-platform=starboard
...
[       OK ] ContentMainRunnerImplBrowserTest.StartupSequence (50 ms)
[----------] 1 test from ContentMainRunnerImplBrowserTest (50 ms total)

[----------] Global test environment tear-down
[==========] 1 test from 1 test suite ran. (50 ms total)
[  PASSED  ] 1 test.

Issue: 433354983

[github-actions]

🤖 Gemini Suggested Commit Message

---

starboard: Enable browser tests on Linux with Starboard

This commit enables the cobalt_browsertests target to run on Linux
when the is_starboard GN argument is set to true. It introduces a new
main entry point for the test runner, integrating the ui/ozone/platform/
starboard library, and adjusts ContentMainDelegate and ContentClient
lifecycles to properly support the test environment. The
ContentMainRunnerImplBrowserTest is plumbed as an initial demo test.

This change expands automated testing coverage for Cobalt on the
Linux/Starboard platform, ensuring the stability and correctness of the
Starboard abstraction layer and its integration with Cobalt.

Bug: 433354983

---

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[gemini-code-assist]

Code Review

This pull request enables cobalt_browsertests on Linux with Starboard, which is a valuable addition. The changes are mostly correct and follow the right direction by updating build files, test configurations, and adding a new main entry point for Starboard. However, I've identified a few areas for improvement. There's a leftover debug log, some code hygiene issues like an unused include and duplicated includes, a potential correctness issue with a missing override specifier, and a memory leak in the new test runner. Addressing these points will enhance the code's quality and maintainability.