[CSS Overview] Fixes color values of 'none'
[CSS Overview] Fixes color values of 'none'

We currently try and parse computed color values from Blink to
power the CSS Overview. A possible value for the computed value
is 'none', which breaks the capture. This CL skips over 'none'
values since Common.Color will not parse it, and will therefore
return null.

Bug: 1018593
Change-Id: I2c8adb6352c21d52500e0a558ed0627c5824c580
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/1881557
Reviewed-by: Tim van der Lippe <dba8716ee7f8d16236046f74d2167cb94410f6ed@chromium.org>
Commit-Queue: Paul Lewis <8915f5d5c39a08c5a3e8a8d7314756e4580816f9@chromium.org>

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