Add --expose-gc to Chrome's flags in Karma launcher configuration.

[jodator]

Suggested merge commit message (convention)

Other: Add '--js-flags="--expose-gc"' flag to Karma Chrome launcher configuration. See ckeditor/ckeditor5#1341.

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Additional information

• This change is required for memory leak tests: Memory leaks fix, part I: create/destroy ckeditor5-core#155.

[coveralls]

Coverage remained the same at 89.147% when pulling 4eb57fd on t/ckeditor5/1341 into 4a2bbb9 on master.

[jodator]

@Reinmar & @pomek This PR shouldn't break anything so it would be nice to merge it and release a package for it so it will be available on Travis for mentioned PR in editor-core.

[pomek]

What about the CI? There is a second Chrome (CHROME_TRAVIS_CI) for CI only. Today you mentioned about the CI so maybe should we add it to both browsers?