[DevTools] removeFrameAttribution should be safer

uiSourceCode may have no attribution, e.g. in case when there is no
frames in Node.js environment.
addFrameAttribution and cloneInitialFrameAttribution check frame or
attribution for existing, let's do the same in removeFrameAttribution.

R=einbinder@chromium.org
TBR=pfeldman@chromium.org

Bug: none
Change-Id: I373d4faaa40210a9445a1f3cc4ea1abb72ec72db
Reviewed-on: https://chromium-review.googlesource.com/1123708
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Reviewed-by: Joel Einbinder <einbinder@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572357}

third_party/blink/renderer/devtools/front_end/bindings/NetworkProject.js

@@ -106,6 +106,8 @@ Bindings.NetworkProject = class {
    */
   static removeFrameAttribution(uiSourceCode, frameId) {
     const frameAttribution = uiSourceCode[Bindings.NetworkProject._frameAttributionSymbol];
+    if (!frameAttribution)
+      return;
     const attributionInfo = frameAttribution.get(frameId);
     console.assert(attributionInfo, 'Failed to remove frame attribution for url: ' + uiSourceCode.url());
     attributionInfo.count -= 1;