Fuchsia: modify Fuchsia runner GN templs to emit exe's runtime_deps.

The prior implementation would emit a runtime_deps manifest for the
fuchsia_executable_runner target by itself, which is minimal and
not useful.

With this CL, the user need only build the fuchsia_executable_runner
target to build the target executable, generate the runner scripts,
and generate the runtime deps file for the target executable itself.


Bug: 
Change-Id: I81d1b5a53615af457fa8bfaaaab49caf80d9d80c
Reviewed-on: https://chromium-review.googlesource.com/642053
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Reviewed-by: Scott Graham <scottmg@chromium.org>
Commit-Queue: Scott Graham <scottmg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#498502}

• BUILD.gn[diff]
• build/config/fuchsia/rules.gni[diff]
• headless/BUILD.gn[diff]