Modify CrashGenerator::CreateChildCrash to copy proc files.

Modify CrashGenerator::CreateChildCrash to copy proc files.

This patch is taken from the downstream version of breakpad in
Chromium OS:  https://gerrit.chromium.org/gerrit/15148

LinuxCoreDumperTest previously assumes the proc files of the child
process created by CrashGenerator::CreateChildCrash() have the same
content as its parent process, which may not be true. This CL modifies
CrashGenerator to copy the proc files of the child process, created by
CreateChildCrash(), before crashing that process.

BUG=chromium-os:25252
TEST=Verified the following:
TEST=Tested the following:
1. Build on 32-bit and 64-bit Linux with gcc 4.4.3 and gcc 4.6.
2. Build on Mac OS X 10.6.8 with gcc 4.2 and clang 3.0 (with latest gmock).
3. All unit tests pass.
Committed: https://code.google.com/p/google-breakpad/source/detail?r=925

[Ben Chan]

This patch is same as:
https://gerrit.chromium.org/gerrit/15148

[Michael Krebs]

LGTM