Change CHECK to return false

This branch is suddenly getting hit, which should never happen, but
misbehaving js is better than a browser crash

Test: None
Bug: 138416788
Change-Id: I3f81ac019b2d0b361bf390f0a1dfce1555a0e392
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1730794
Commit-Queue: Albert Chaulk <achaulk@chromium.org>
Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683239}

chromecast/browser/extensions/api/tabs/tabs_api.cc

@@ -762,8 +762,12 @@ ExecuteCodeFunction::InitResult ExecuteCodeInTabFunction::Init() {
 
 bool ExecuteCodeInTabFunction::CanExecuteScriptOnPage(std::string* error) {
   const CastWebContents* webview = GetWebViewForTab(execute_tab_id_);
+  if (!webview) {
+    *error = ErrorUtils::FormatErrorMessage(
+        keys::kTabIndexNotFoundError, base::NumberToString(execute_tab_id_));
+    return false;
+  }
 
-  CHECK(webview);
   content::WebContents* contents = webview->web_contents();
 
   int frame_id = details_->frame_id ? *details_->frame_id