Synthetics - IsCritical missing on SyntheticsStep

[mattbartel-tyler]

SyntheticsStep doesn't have IsCritical even though the API is returning this value (compared to SyntheticsApiStep which does).

Expected behavior

`IsCritical' should be available on both API and browser tests.

Environment and Versions (please complete the following information):

Version 1.8.0

[therve]

Thanks for your report, should be fixed by #1327.