RC: Allow renderer-initiated freezing, as triggered by ChromeDriver

Change TabLifecycleUnit to consider active --> frozen as a valid state
change if initiated from the renderer. Freezing a page through
ChromeDriver will trigger a renderer-initiated freeze event and cause
this transition. This is necessary for wpt tests to work on
DCHECK-enabled builds outside of web_tests.

Bug: 930270
Change-Id: I335ba03eba6046e397c4917561c556eb72216a99
Reviewed-on: https://chromium-review.googlesource.com/c/1460433Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: Scott Haseley <shaseley@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630568}

chrome/browser/resource_coordinator/tab_lifecycle_unit.cc

@@ -71,6 +71,11 @@ bool IsValidStateChange(LifecycleUnitState from,
           return reason == StateChangeReason::SYSTEM_MEMORY_PRESSURE ||
                  reason == StateChangeReason::EXTENSION_INITIATED;
         }
+        case LifecycleUnitState::FROZEN: {
+          // Render-initiated freezing, which happens when freezing a page
+          // through ChromeDriver.
+          return reason == StateChangeReason::RENDERER_INITIATED;
+        }
         default:
           return false;
       }