perf_to_profile drops perf symbols

perf_to_profile drops symbol names when converting from perf output to pprof format.

```
$ perf record tree /usr
$ perf report
Samples: 4K of event 'cpu-clock:uhH', Event count (approx.): 1236250000
Overhead Command Shared Object Symbol
12.52% tree tree [.] 0x000000000000c964
11.06% tree libc-2.27.so [.] vfprintf
4.63% tree libc-2.27.so [.] write
4.33% tree libc-2.27.so [.] __lxstat64
3.84% tree libc-2.27.so [.] _IO_file_xsputn
3.01% tree libc-2.27.so [.] __fprintf_chk
2.75% tree libc-2.27.so [.] __strcoll_l
2.43% tree libc-2.27.so [.] c32rtomb
1.92% tree libc-2.27.so [.] malloc
1.80% tree libc-2.27.so [.] 0x000000000016af98
1.72% tree libc-2.27.so [.] 0x00000000000e00a8
1.62% tree libc-2.27.so [.] 0x000000000018e5a5
1.54% tree libc-2.27.so [.] cfree
1.38% tree libc-2.27.so [.] iswprint
1.01% tree libc-2.27.so [.] 0x000000000018e590
0.87% tree libc-2.27.so [.] __open_nocancel
0.87% tree libc-2.27.so [.] 0x0000000000169d90
0.75% tree libc-2.27.so [.] __close_nocancel
0.75% tree libc-2.27.so [.] __fxstat64
0.75% tree libc-2.27.so [.] readdir64
0.69% tree libc-2.27.so [.] 0x0000000000169d7a
0.69% tree libc-2.27.so [.] 0x000000000018e59c
0.67% tree libc-2.27.so [.] 0x0000000000169d98
0.67% tree libc-2.27.so [.] 0x000000000018e230
0.65% tree libc-2.27.so [.] 0x000000000018e69d
0.61% tree tree [.] 0x000000000000d672
0.59% tree tree [.] 0x0000000000007bbe
0.57% tree libc-2.27.so [.] _IO_default_xsputn
0.53% tree libc-2.27.so [.] 0x000000000018e690
0.51% tree tree [.] 0x000000000000c949
0.51% tree tree [.] 0x000000000000c95c
0.44% tree libc-2.27.so [.] 0x0000000000169d6c
0.42% tree libc-2.27.so [.] 0x000000000018e22b
0.42% tree tree [.] 0x000000000000d675
0.40% tree libc-2.27.so [.] 0x00000000000e00b0
0.40% tree libc-2.27.so [.] 0x000000000016af9e
0.38% tree libc-2.27.so [.] _IO_file_overflow
0.38% tree libc-2.27.so [.] 0x000000000018e227
0.36% tree libc-2.27.so [.] _IO_file_write
0.36% tree libc-2.27.so [.] mbstowcs
0.34% tree libc-2.27.so [.] 0x0000000000169d60
0.34% tree libc-2.27.so [.] 0x0000000000169de5
0.32% tree libc-2.27.so [.] _dl_mcount_wrapper_check
...
```

After conversion pprof data only contains the "Shared Object" name pprof accumulates all profiles under shared object names instead of symbols.
```
(pprof) top
Showing nodes accounting for 1236000000, 100% of 1236250000 total
Dropped 1 node (cum <= 6181250)
flat flat% sum% cum cum%
956250000 77.35% 77.35% 956250000 77.35% [libc-2.27.so]
279750000 22.63% 100% 279750000 22.63% [tree]
```

Start at the perf_to_profile conversion entry point and reproduce the issue with the perf report shown here. Trace how each Shared Object and Symbol column is represented in the profile.proto output. Done means pprof separates samples by symbol name rather than accumulating them only under shared object names.

Written by the indexing model from the issue text.