Automatically run file command on remote connection to the target if NaCl manifest was provided by …

Automatically run file command on remote connection to the target if NaCl
manifest was provided by nacl-manifest command before that.

All information about NaCl program is already in the manifest. So we shouldn't
require to enter this information twice.

We can't just execute file command whenever user issues nacl-manifest command
though since we don't know the program architecture until gdb will connect to
debug stub. That's why this should be done the moment we know the program
architecture (and file command doesn't fail) but before gdb tries to find
symbols in the main program. The only such moment when we can execute our code
is about_to_proceed event.

BUG= http://code.google.com/p/nativeclient/issues/detail?id=2848
TEST= run chrome with --enable-nacl-debug --nacl-gdb=..., set breakpoints,
execute target remote command.

R=eaeltsin@google.com

Committed: https://git.chromium.org/gitweb?p=native_client/nacl-gdb.git;a=commit;h=183c0d2

[eaeltsin, 2012-06-13 19:25:22]

lgtm

Not sure I like the way our gdb initializes, but don't want to block browser
integration on this.

We might want to refactor this later. The main problem is that gdb does not know
the architecture before connecting to the target, and after connecting might be
too late for a number of setup hooks.