Documentation - Remove references to `gclient sync`

skia.org/user/download
-   Move sections "Making changes" and "Uploading changes for review"
    to "How to submit a patch" page
-   Mention bin/sync-and-gyp

skia.org/dev/contrib/submit
-   Mention branch.autosetuprebase=always to fit with Skia's linear
    history style.

NOTRY=true
DOCS_PREVIEW= https://skia.org/user/download?cl=1411403010
DOCS_PREVIEW= https://skia.org/dev/contrib/submit?cl=1411403010

Review URL: https://codereview.chromium.org/1411403010
2 files changed