Fix ContentBrowserTest.BrowserCrashCallStack in Linux ThinLTO tests

The test was failing due to
ContentBrowserTest_MANUAL_BrowserCrash_Test::RunTestOnMainThread
not appearing in the stack trace in ThinLTO builds (link below).
Add pragmas to prevent the compiler from optimizing it away.

https://ci.chromium.org/ui/p/chromium/builders/ci/ToTLinuxThinLTO/13086

Bug: 1155142
Change-Id: I191e7642f65c5f2dc17cccef449ca0746e31922d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2566968
Auto-Submit: Hans Wennborg <hans@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833399}

content/test/content_browser_test_test.cc

@@ -145,9 +145,16 @@ IN_PROC_BROWSER_TEST_F(ContentBrowserTest, RendererCrashCallStack) {
   }
 }
 
+#ifdef __clang__
+// Don't optimize this out of stack traces in ThinLTO builds.
+#pragma clang optimize off
+#endif
 IN_PROC_BROWSER_TEST_F(ContentBrowserTest, MANUAL_BrowserCrash) {
   CHECK(false);
 }
+#ifdef __clang__
+#pragma clang optimize on
+#endif
 
 // Tests that browser tests print the callstack on asserts.
 // Disabled on Windows crbug.com/1034784