RC: Allow a tab pending discard to be urgently discarded.

There is no reason to prevent this.

Change-Id: Ice12038054b3ffcbf1e165017a8c1238fb84670e
Reviewed-on: https://chromium-review.googlesource.com/c/1371934Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615906}

chrome/browser/resource_coordinator/tab_lifecycle_unit.cc

@@ -124,7 +124,9 @@ bool IsValidStateChange(LifecycleUnitState from,
         //   - The freeze timeout expires, or,
         //   - The renderer notifies the browser that the page has been frozen.
         case LifecycleUnitState::DISCARDED:
-          return reason == StateChangeReason::BROWSER_INITIATED;
+          return reason == StateChangeReason::BROWSER_INITIATED ||
+                 reason == StateChangeReason::SYSTEM_MEMORY_PRESSURE ||
+                 reason == StateChangeReason::EXTENSION_INITIATED;
         // The WebContents is focused.
         case LifecycleUnitState::PENDING_FREEZE:
           return reason == StateChangeReason::USER_INITIATED;