Work on platform GN build.

Add a source set for the heap directory, fill out more dependencies of the blink platform target that have been converted already.

Rename the blink_platform target "platform" so you can refer to it by directory name only, and use the output_name to avoid ambiguity when doing a component build.

BUG=

Review URL: https://codereview.chromium.org/270383008

git-svn-id: svn://svn.chromium.org/blink/trunk@174039 bbb929c8-8fbe-4397-9dbb-9b2b20218538