Fix GLHelperReadbackSupport::SupportsFormat crash

GetString(GL_EXTENSIONS) can return NULL if the context was lost. Handle
gracefully.

BUG=421608

Review URL: https://codereview.chromium.org/639073003

Cr-Commit-Position: refs/heads/master@{#298759}

content/common/gpu/client/gl_helper_readback_support.cc

@@ -106,10 +106,12 @@ bool GLHelperReadbackSupport::SupportsFormat(GLenum format, GLenum type) {
 
   if (format == GL_BGRA_EXT && type == GL_UNSIGNED_BYTE) {
     const GLubyte* tmp = gl_->GetString(GL_EXTENSIONS);
-    std::string extensions =
-        " " + std::string(reinterpret_cast<const char*>(tmp)) + " ";
-    if (extensions.find(" GL_EXT_read_format_bgra ") != std::string::npos) {
-      return true;
+    if (tmp) {
+      std::string extensions =
+          " " + std::string(reinterpret_cast<const char*>(tmp)) + " ";
+      if (extensions.find(" GL_EXT_read_format_bgra ") != std::string::npos) {
+        return true;
+      }
     }
   }