It's been pointed out by Evan Parker from the MapsGL team that getExtension("WEBGL_compressed_texture_s3tc") is returning a non-null value even when the underlying hardware doesn't support the extension. A supports() check in getExtension is missing.
Created attachment 187716 [details]
Comment on attachment 187716 [details]
Thanks for the quick fix. Looks good. Note for the record that this will eventually be covered by the WebGL conformance tests Gregg Tavares is in the process of importing.
Attachment 187716 [details] did not pass style-queue:
Failed to run "['Tools/Scripts/check-webkit-style', '--diff-files', u'Source/WebCore/ChangeLog', u'Source/WebCore/html/canvas/WebGLRenderingContext.cpp']" exit_code: 1
Source/WebCore/html/canvas/WebGLRenderingContext.cpp:2409: Weird number of spaces at line-start. Are you using a 4-space indent? [whitespace/indent] 
Source/WebCore/html/canvas/WebGLRenderingContext.cpp:2410: Weird number of spaces at line-start. Are you using a 4-space indent? [whitespace/indent] 
Total errors found: 2 in 2 files
If any of these errors are false positives, please file a bug against check-webkit-style.
Committed r142545: <http://trac.webkit.org/changeset/142545>