WebUI testing: Fix inaccurate TestBrowserProxy#getCallCount signature.

Bug: None
Change-Id: I68574edf3f94422f9dc87edb9a31fc29b163483f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2099027
Commit-Queue: dpapad <dpapad@chromium.org>
Commit-Queue: John Lee <johntlee@chromium.org>
Auto-Submit: dpapad <dpapad@chromium.org>
Reviewed-by: John Lee <johntlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#749383}

chrome/test/data/webui/test_browser_proxy.js

@@ -99,7 +99,7 @@ let MethodData;
   /**
    * Get number of times method is called.
    * @param {string} methodName
-   * @return {!boolean}
+   * @return {number}
    */
   getCallCount(methodName) {
     return this.getMethodData_(methodName).callCount;