GeneratorGeneratedImage should be GradientGeneratedImage

Blink r150482 [1] removed the concept of a "Generator", making
GeneratorGeneratedImage require a Gradient but it retained the
original and totally unrelated name: so s/Generator/Gradient/g
to create GradientGeneratedImage similar to [2].

[1] https://src.chromium.org/viewvc/blink?view=revision&revision=150482
[2] https://bugs.webkit.org/show_bug.cgi?id=121085

BUG=None

Review URL: https://codereview.chromium.org/26108005

git-svn-id: svn://svn.chromium.org/blink/trunk@159883 bbb929c8-8fbe-4397-9dbb-9b2b20218538