[fuchsia] Fix a crash due to uninitialized member.

A bug was introduced in https://crrev.com/c/2355924 leading to
|semantics_manager_for_test_| being left unitialized, resulting in a
crash in release builds.

Bug: 1117070
Change-Id: I249c232e616145a21edf380077bcd58a1571c085
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2360315
Auto-Submit: Fabrice de Gans-Riberi <fdegans@chromium.org>
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798807}

fuchsia/engine/browser/frame_impl.h

@@ -246,7 +246,7 @@ class FrameImpl : public fuchsia::web::Frame,
 
   std::unique_ptr<AccessibilityBridge> accessibility_bridge_;
   fuchsia::accessibility::semantics::SemanticsManager*
-      semantics_manager_for_test_;
+      semantics_manager_for_test_ = nullptr;
 
   EventFilter event_filter_;
   NavigationControllerImpl navigation_controller_;