fuchsia: Return ModuleSnapshot* out of ProcessSnapshotFuchsia
fuchsia: Return ModuleSnapshot* out of ProcessSnapshotFuchsia

And document that UnloadedModules() isn't applicable on Fuchsia.

Bug: crashpad:196
Change-Id: Ic2c5f26fbc9cbd908ec0b941797c63f88caeec9c
Reviewed-on: https://chromium-review.googlesource.com/929302
Reviewed-by: Mark Mentovai <f1b5a91d4d6ad523f2610114591c007e75d15084@chromium.org>
Commit-Queue: Scott Graham <e134406e1863f3b48b0795831afac5c0595efb36@chromium.org>

chromium/crashpad,chromium/crashpad,chromium/crashpad