[Fuchsia] Fix ContextImpl::DestroyFrame() to search the frame only once

Previously the frame to be deleted was search in |frames_| twice, which
is not necessary.

Change-Id: Ieb0df172ef6c04249d2c44588a932d28f5158908
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1986190
Auto-Submit: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#728047}

fuchsia/engine/browser/context_impl.cc

@@ -47,7 +47,7 @@ ContextImpl::~ContextImpl() {
 void ContextImpl::DestroyFrame(FrameImpl* frame) {
   auto iter = frames_.find(frame);
   DCHECK(iter != frames_.end());
-  frames_.erase(frames_.find(frame));
+  frames_.erase(iter);
 }
 
 bool ContextImpl::IsJavaScriptInjectionAllowed() {