Add field-initializing constructors to generated mojo structs.

Currently, when creating mojo-generated struct instances, it requires a
multi-step process, e.g.:
FooPtr foo = Foo::New();
foo->bar = bar;
foo->baz = baz;

This leads to helper factory functions of the form:
FooPtr MakeFoo(int bar, int baz) {
  FooPtr foo = Foo::New();
  foo->bar = bar;
  foo->baz = baz;
  return foo;
}
with call sites of the form:
FooPtr foo = MakeFoo(bar, baz);

This boilerplate should not be necessary. This CL adds constructors
to generated mojo structs that initialize struct fields, allowing the
simpler:
FooPtr foo = Foo::New(bar, baz);

CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Review-Url: https://codereview.chromium.org/2689513003
Cr-Commit-Position: refs/heads/master@{#453494}
26 files changed