Restore severity precondition to logging.h.
Restore severity precondition to logging.h.

I mistakenly ommitted the checks when logging.h was ported from
libjingle to webrtc. This caused a significant CPU cost for logs which
were later filtered out anyway.

Verified with LS_VERBOSE logging in neteq4, running:
$ out/Release/modules_unittests \
--gtest_filter=NetEqDecodingTest.TestBitExactness \
--gtest_repeat=50 > time.txt
$ grep "case ran" time.txt | grep "[0-9]* ms" -o | sort

Results on a MacBook Retina, averaged over 5 runs:
Verbose logs disabled:                          666 ms
Exisiting implementation, verbose logs enabled: 944 ms (1.42x)
New implementation, verbose logs enabled:       673 ms (1.01x)

BUG=2314
R=henrik.lundin@webrtc.org, henrike@webrtc.org, kjellander@webrtc.org, turaj@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/2160005

git-svn-id: a556c8bde68cedbc0208a1ffc78b06e9cd07f676@4682 4adac7df-926f-26a2-2b94-8c16560cd09d

bpsinc-native/src_third_party_libjingle_source_talk,geekboxzone/lollipop_external_chromium_org_third_party_libjingle_source_talk,dcpang/fatline-libjingle,xin3liang/platform_external_chromium_org_third_party_libjingle_source_talk,dcpang/fatline-libjingle,dcpang/fatline-libjingle,sippet/talk,xin3liang/platform_external_chromium_org_third_party_libjingle_source_talk,bpsinc-native/src_third_party_libjingle_source_talk,AOSPU/external_chromium_org_third_party_libjingle_source_talk,sippet/talk,bpsinc-native/src_third_party_libjingle_source_talk,sippet/talk,Omegaphora/external_chromium_org_third_party_libjingle_source_talk,Omegaphora/external_chromium_org_third_party_libjingle_source_talk,xin3liang/platform_external_chromium_org_third_party_libjingle_source_talk,android-ia/platform_external_chromium_org_third_party_libjingle_source_talk,dcpang/fatline-libjingle,geekboxzone/lollipop_external_chromium_org_third_party_libjingle_source_talk,xin3liang/platform_external_chromium_org_third_party_libjingle_source_talk,bpsinc-native/src_third_party_libjingle_source_talk,sippet/talk,geekboxzone/lollipop_external_chromium_org_third_party_libjingle_source_talk,android-ia/platform_external_chromium_org_third_party_libjingle_source_talk,AOSPU/external_chromium_org_third_party_libjingle_source_talk,geekboxzone/lollipop_external_chromium_org_third_party_libjingle_source_talk,Omegaphora/external_chromium_org_third_party_libjingle_source_talk,AOSPU/external_chromium_org_third_party_libjingle_source_talk,bpsinc-native/src_third_party_libjingle_source_talk,dcpang/fatline-libjingle,AOSPU/external_chromium_org_third_party_libjingle_source_talk,android-ia/platform_external_chromium_org_third_party_libjingle_source_talk,xin3liang/platform_external_chromium_org_third_party_libjingle_source_talk,Omegaphora/external_chromium_org_third_party_libjingle_source_talk,Omegaphora/external_chromium_org_third_party_libjingle_source_talk,sippet/talk,android-ia/platform_external_chromium_org_third_party_libjingle_source_talk,bpsinc-native/src_third_party_libjingle_source_talk,AOSPU/external_chromium_org_third_party_libjingle_source_talk,geekboxzone/lollipop_external_chromium_org_third_party_libjingle_source_talk,android-ia/platform_external_chromium_org_third_party_libjingle_source_talk,Omegaphora/external_chromium_org_third_party_libjingle_source_talk,android-ia/platform_external_chromium_org_third_party_libjingle_source_talk