Do not build h264 on Linux32 Debug bots.

Setting ffmpeg_branding = "Chrome" is what causes a compilation error
(see [1], where h264_cabac.c is included):
../../third_party/ffmpeg/libavcodec/x86/cabac.h:193:9: error: inline assembly requires more registers than available
        BRANCHLESS_GET_CABAC("%0", "%q0", "(%4)", "%1", "%w1",
[...]

See: https://build.chromium.org/p/tryserver.webrtc/builders/linux32_dbg/builds/50.


Also, from the compililation error, this might be a bug on clang?

...
clang: note: diagnostic msg: PLEASE submit a bug report to http://llvm.org/bugs/ and include the crash backtrace, preprocessed source, and associated run script.
...


[1] https://cs.chromium.org/chromium/src/third_party/ffmpeg/ffmpeg_generated.gni?l=222


Bug: webrtc:7413
No-Try: true
Change-Id: I5c785d8f6f61c72a5e7665367023fec017b18d3e
Reviewed-on: https://webrtc-review.googlesource.com/37360
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Edward Lemur <ehmaldonado@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21489}

tools_webrtc/mb/mb_config.pyl

@@ -35,7 +35,7 @@
       'Mac64 Release [large tests]': 'release_bot_x64',
 
       # Linux
-      'Linux32 Debug': 'debug_bot_x86',
+      'Linux32 Debug': 'no_h264_debug_bot_x86',
       'Linux32 Release': 'release_bot_x86',
       'Linux32 Debug (ARM)': 'debug_bot_arm',
       'Linux32 Release (ARM)': 'release_bot_arm',
@@ -165,7 +165,7 @@
       'linux_rel': 'release_bot_x64',
       'linux_arm64_dbg': 'debug_bot_arm64',
       'linux_arm64_rel': 'release_bot_arm64',
-      'linux32_dbg': 'debug_bot_x86',
+      'linux32_dbg': 'no_h264_debug_bot_x86',
       'linux32_rel': 'release_bot_x86',
       'linux32_arm_dbg': 'debug_bot_arm',
       'linux32_arm_rel': 'release_bot_arm',
@@ -255,6 +255,9 @@
     'debug_bot_x86': [
       'gn', 'openh264_debug_bot', 'x86'
     ],
+    'no_h264_debug_bot_x86': [
+      'gn', 'debug_bot', 'x86'
+    ],
     'release_bot_x86': [
       'gn', 'openh264_release_bot', 'x86'
     ],