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According to W3C spec, new session capabilities with null values should
be ignored, but they cause errors in ChromeDriver. This CL fixes it.

See https://www.w3.org/TR/webdriver/#dfn-validate-capabilities, step 3,
sub-steps 3 and 4.

Bug: chromedriver:1997
Change-Id: Ibee5e383c46139df1e5efc9f8a46872820c81046
Reviewed-on: https://chromium-review.googlesource.com/c/1340834
Commit-Queue: John Chen <johnchen@chromium.org>
Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609069}