[bgmode] Log the unpin stack trace to Crash Keys

chrome/common/crash_keys.cc

Index: chrome/common/crash_keys.cc
diff --git a/chrome/common/crash_keys.cc b/chrome/common/crash_keys.cc
index e98a5e9831c11d2bddd2c79cb31ba0fd166b0780..63eb8a562a4e3b37e48ab4fa871eeb301588deb1 100644
--- a/chrome/common/crash_keys.cc
+++ b/chrome/common/crash_keys.cc
@@ -33,6 +33,7 @@ const char kExtensionID[] = "extension-%" PRIuS;
 const char kNumExtensionsCount[] = "num-extensions";
 
 const char kShutdownType[] = "shutdown-type";
+const char kBrowserUnpinTrace[] = "browser-unpin-trace";
 
 #if !defined(OS_ANDROID)
 const char kGPUVendorID[] = "gpu-venid";
@@ -118,6 +119,7 @@ size_t RegisterChromeCrashKeys() {
     { kVariations, kLargeSize },
     { kNumExtensionsCount, kSmallSize },
     { kShutdownType, kSmallSize },
+    { kBrowserUnpinTrace, kMediumSize },
 #if !defined(OS_ANDROID)
     { kGPUVendorID, kSmallSize },
     { kGPUDeviceID, kSmallSize },