Add a new gulp task for patchWebAnimations

[rsimha]

#13199 moved the call to patchWebAnimations() from the global scope in gulpfile.js to performBuild(), which is called by build() and watch(). Then, #13221 added a call to dist(). Turns out we need the call in gulp dep-check as well.

This PR adds a new gulp patch-web-animations task that's used as a prerequisite for gulp, gulp build, gulp watch, gulp dist, and gulp dep-check

See https://travis-ci.org/ampproject/amphtml/jobs/336675870#L1736 for the failure case.

Follow up to #13199 and #13221
Fixes #13177
Partial fix for #13227

[rsimha]

/to @choumx

Tested locally by deleting node_modules and running gulp clean && gulp dep-check. It used to fail, but now passes.

[dreamofabear]

Does dep-check run compileCss()? I don't see the reference in dep-check.js.

[rsimha]

It does, via a dependency expressed in https://github.com/ampproject/amphtml/blob/master/build-system/tasks/dep-check.js#L300

[dreamofabear]

Ah, thanks. Might be more readable to import patchWebAnimations directly into dep-check? I think web animations semantically are not related to CSS.

[rsimha]

Good point. I've moved the function into its own gulp task and used it as a dependency wherever it's needed. Much cleaner now.

[rsimha]

/cc @erwinmombay FYI