Define WEBRTC_CHROMIUM_BUILD in libjingle.gyp.

Code under webrtc/ expects this to be defined when building against
Chromium. With talk/base having moved to webrtc/base, libjingle code
must adopt this expectation. This appears to be the most straightforward
way of adding this define.

In particular, this is now required to avoid redefinition errors after
the addition of CHECK macros to webrtc/base.

TESTED=local Linux build against the latest webrtc passes.

Review URL: https://codereview.chromium.org/516143002

Cr-Commit-Position: refs/heads/master@{#292676}

third_party/libjingle/libjingle.gyp

@@ -31,6 +31,7 @@
       'SRTP_RELATIVE_PATH',
       'USE_WEBRTC_DEV_BRANCH',
       'ENABLE_EXTERNAL_AUTH',
+      'WEBRTC_CHROMIUM_BUILD',
     ],
     'configurations': {
       'Debug': {