Simplify tracing code when tracing is disabled.

[m0hamed]

If tracing is disabled, instead of using hb_auto_trace_t<0, ret_t>, a
new struct is used instead hb_no_trace<ret_t>. Using the simpler struct
reduces the binary size for release builds by removing unneaded debug
code.

These are the results of the difference (from chromium build with and without patch):
Section Sizes (Total=-101kb (-103544 bytes)):
.bss: 0 bytes (0 bytes) (not included in totals)
.data: 0 bytes (0 bytes) (-0.0%)
.data.rel.ro: 0 bytes (0 bytes) (-0.0%)
.rel.dyn: 0 bytes (0 bytes) (-0.0%)
.rodata: -73.5kb (-75264 bytes) (72.7%)
.text: -27.6kb (-28280 bytes) (27.3%)

MonochromePublic.apk_Breakdown (-106,496 bytes)
+2 bytes Zip Overhead
-106,496 bytes Native code size
-2 bytes Package metadata size
MonochromePublic.apk_Specifics
-106,496 bytes normalized apk size
-106,496 bytes main lib size

For reference the compiled library size in full is 247,609 bytes, in the chrome for android apk.

[m0hamed]

Here is the chromium bug about the issue.

[behdad]

Thanks Mohamed. We noticed this independently as well, but only on Android with clang. To me, this looks like a clang bug. I'll comment in the Chromium bug.