Generate common_custom_types__type_mappings on build time.

Instead of checking in pre-generated files,
this CL generates the file on build time.

Note that gen/mojo/common/common_custom_types__type_mappings file
is not yet removed. It is necessary to wait for internal catch up.

Bug: 79557560
Test: Built. No diff between generated file and pre-generated file.
Change-Id: Ia285fda93242216c8149d2871406235574e6258b
2 files changed