[bgmode] Log the unpin stack trace to Crash Keys

chrome/common/crash_keys.h

Index: chrome/common/crash_keys.h
diff --git a/chrome/common/crash_keys.h b/chrome/common/crash_keys.h
index f22d0f4d4ddc57dfdf95581fe2743983f4f854d5..3a811f7ee23b214a049a76ec85f01944a769a864 100644
--- a/chrome/common/crash_keys.h
+++ b/chrome/common/crash_keys.h
@@ -68,6 +68,11 @@ extern const char kNumExtensionsCount[];
 // "exit" for BROWSER_EXIT, or "end" for END_SESSION.
 extern const char kShutdownType[];
 
+// Stack trace associated to the browser being unpinned and starting the
+// shutdown sequence. The value is set when we trigger a browser crash due to an
+// invalid attempt to Pin the browser process after that.
+extern const char kBrowserUnpinTrace[];
+
 // GPU information.
 #if !defined(OS_ANDROID)
 extern const char kGPUVendorID[];