[ChromeDriver] Enable testShouldCaptureScreenshotOfCurrentViewport on Mac

Disabling testShouldCaptureScreenshotOfCurrentViewport was done by mistake
when we run the test on local machine without "--chrome" flag.
This test must be run with the flag "--chrome=<path_to_chrome_binaries>"

Bug: chromedriver:2480
Change-Id: I21cc8c155a1eec1952eb3e50b2dcd27a8aa450cb
Reviewed-on: https://chromium-review.googlesource.com/1238825Reviewed-by: John Chen <johnchen@chromium.org>
Commit-Queue: Artur Khachatryan <khachatryan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593263}

chrome/test/chromedriver/test/test_expectations

@@ -32,9 +32,6 @@ _OS_NEGATIVE_FILTER['win'] = [
 _OS_NEGATIVE_FILTER['linux'] = [
 ]
 _OS_NEGATIVE_FILTER['mac'] = [
-    # https://bugs.chromium.org/p/chromedriver/issues/detail?id=2480
-    'TakesScreenshotTest.testShouldCaptureScreenshotOfCurrentViewport',
-
     # https://crbug.com/868599
     'WindowTest.testCanMaximizeTheWindowFromFrame',
     'WindowTest.testCanMaximizeTheWindowFromIframe',